That's a pretty weak analogy.
Trademark law is a very convoluted thing. Most intellectual property law is, in fact. And there is no guarantee that a trademark is valid just because you registered it. This is for a wide variety of reasons, but the basic idea of trademarking is to protect your trade dress -- the things that identify you as a company -- and to stop confusion in the marketplace. The fundamental question of a design that is trademarked isn't necessarily answered by registration of the trademark, and trademarks can be challenged for a great variety of reasons including that someone could easily trademark something super generic to lock up something other people are doing, or could trademark a design simply to stop it from being produced. Trademarks must be defended to be valid, and must be constantly defended.
Your analogy is akin to saying Dean took a Gibson guitar, depriving Gibson of that guitar. They did no such thing. Their producing their own guitars only competes, this is not stealing. It wasn't a violation of a registered trademark when they started (Dean started the company in 1977 making a V, which wasn't trademarked by Gibson until the mid 90s) and it has long been held that headstocks and logos were the defining trade dress of a guitar and body shapes have all been borrowed for inspiration or straight copying as long as there has been a guitar industry.
So, Dean may well have thought they were doing the same thing Gibson was doing when it created a Dreadnought style guitar in response to Martin's reissue of their Dred design. Creating their own version of a generic design with their own twist on it. The 1934 Hummingbird isn't that different than a D2, it just has slightly different finish options. Certain non-patentable technologies or design aesthetics that aren't already trademarked are routinely allowed based on function and generic nature. Guitars have always had that hourglass shape, so even a dred being less hourglassy than a 000 is still a generic shape. Like if you're designing condoms, you really can't stop a competitor for making a penis shaped condom just because your condom is penis shaped. It is a generic thing that is done because the item doesn't work any other way, or that it's super common to solve the problem that way, just as a guitar must have strings a fretboard, a bridge, etc... Maybe the first guy who make a guitar curvey so it could sit on a knee was an innovator, but after enough time, when enough other people had made curvy guitars (which would seem obvious as all orchestral stringed instruments also gained waists) pretty much any similar shape was fair game.
And there's the rub. If something has become generic, even if your grandpappy was the first one to make one, you can't trademark it. Gibson stopped making Vs between the first run in 1958 and 1967. Same with explorers, which they stopped making until OTHER companies started making similar shapes in the mid 70s. You couldn't buy a Gibson Explorer newly produced in 1965, or 1975, and Gibson had NEVER trademarked the shapes or caused them to be identifiers after the 1950s. They started making them again because lots of small luthiers were popping them out. Small timers including Dean and tons of other luthiers had built a market for the Explorer shape. Including Hamer, BTW, started with a V bass and who was making the Standard for four years before Gibson decided to make Explorers again.
STILL, Gibson didn't register a trademark or try to enforce their rights over something like the shape of the flying V. Instead, allowing a world-wide market in Vs and Zs to flourish for 15 years before registering a trademark on the Flying V, and not even bothering to test that trademark in court.
What they DID go after in the early 70s was guitars like Ibanez's 2350, which looked precisely like a Les Paul, down to the font in the logo, headstock shape, inlays and the like, and they never won in court instead reaching an out of court settlement that had Ibanez change it's headstock shape, logo, and a couple of other small details. Note, this was BEFORE they started making the Destroyer. So Z and V shapes started getting made, Gibson never complained, and everyone was doing it so there was no notion that it was wrong.
That's tacit approval.
So, here's the real issue. Not that someone "Stole" anything. Nobody deprived Gibson of the Explorer, as they weren't even making it. And people hadn't thought the V shape was the sole property of Gibson as Gibson didn't say it was, didn't tell anyone they couldn't make it, and didn't even make overtures to do so for 15 more years.
What will need to be determined is whether a V or Z shape is "generic" for guitars, and 50 years of other manufacturers (not just Dean) making similar shaped guitars, the first 35 of them without a peep from Gibson, makes that an uphill battle. Note that Fender lost their trademarks on body shapes because others had been making them and they were declared Generic (P bass, Strat, AND Tele) http://ttabvue.uspto.gov/ttabvue/v?pno=91162497&pty=OPP&eno=13 and Gibson has already lost the V argument in the EU where courts determined the V was unique in 1958, but has since become so ubiquitous as to be generic.
If they lose on the Generic point in the US, his leaves the only card in Gibson's deck the one of Trade Dress and Market Confusion. They'll need to convince a jury that someone looking to purchase such a guitar seeing a Dean V and a Gibson V would confuse the two, and think the Dean was a Gibson. This confusion thing has worked in the past, but it was Headstock shape for Fender, who still retain and protect trademarks on their headstocks. But the typical Dean V shaped headstock and wing logos really work in their favor here. As much as Gibson's open book headstock differentiated the Hummingbird from the Martin Dreds.
And don't miss the fact that Gibson has been talking to Dean's dealer network, threatening them for carrying Dean guitars. Dean HAS to make these defenses, and has the right to call Gibson on illegal anticompetetive behavior to bolster their side. When legal filings are made, nobody gives a shit what simplistic advice your mom gave to a 5 year old you, they must conform to certain legal language so as to force the judge to consider very specific issues in a certain way and often they will be couched in terms that seem uncivil, or might even sound like complete hyperbole. But that's how American jurisprudence works. Maybe someting is always stated as "grave harm" instead of "it hurt a little" because the legal system holds if the harm isn't grave or irreparable then the judge cannot award the same damages. Imagine it this way. If a soccer player who just got kicked in the knee is going to take a dive in front of the ref, he's going to act like his goddamned leg is broken because screaming like Nancy Kerrigan forces the ref to pull out the yellow card if, in fact, he decides it was a foul. Gibson and Dean's posturing here is the same sort of deal. Folksy and imprecise concepts of right and wrong really don't sway anything in court.