Selling old material with old ideals and themes to new current audiences is risky. Even when an old book, show, or play is converted to a movie or TV show, major changes are made to make it interesting and palatable to a new audience.
My little cousin isn't going to buy Classic Dragonlance.
MY niece cousin isn't going to buy Classic Dragonlance.
Restrictions is one thing. But selling a selling is not just a DM piting a setting to 3-6 players. WOTC wants thousands of sales by people who don't have any books in the setting.
The question you're begging there of course 'what IS classic Dragonlance, exactly?' And that's a question with no easy answer. War of the Lance only? Heroes of the Lance era including War of the Twins? What about the Chaos War, or the War of Souls? Dragonlance changed a LOT over the years, including the incorporation of sorcerers etc as they appeared in core D&D. It seems inevitable that even the most conservative 5e incarnation, whether by WotC or someone else, will make changes too. So if you're updating it, how do you know what stuff you can't meddle with and what can be updated without affecting the real core spirit of the setting?
For me, for example, 5e Ravenloft made a big misstep and lost the core spirit of its old setting when it basically decided not to do any worldbuilding and that the vast majority of NPCs are just soulless phantoms sleepwalking about as background scenery in someone else's morality play. For me the horror aspect of Ravenloft is meaningless unless there are innocents and regular people to both protect from darkness, and contrast with it, and having PCs walk around being The Only Really Real People Who Really Matter is uncomfortably sociopathic. But other people liked this design decision because it re-focused domains as being entirely centred around the darklord with no extraneous stuff muddying the waters or people asking annoyingly prosaic questions about 'where is the opera house that Strahd wears his opera cloak to anyway? And who made it? And where did they get the silk?' Of course, those people are objectively wrong...
Same with Dark Sun. Muls for instance - back in the original line, they were bred as gladiator slaves by sorcerer kings from human women and dwarf men (but never the other way around for some reason?) which inevitably resulted in the death of the human mother in childbirth. Now this is icky for a number of different reasons, but it was the early 90s and grimdark was in, so whatever. But removing it completely and just re-defining muls as regular half-dwarves without the baggage doesn't actually change the fundamentals of the setting in any major way. So this is low-hanging fruit for a 5e update, in my opinion. However, Dark Sun also has PC templars (who are literally the corrupt enforcers of a brutal dictatorial slave state) and widespread chattel slavery as a setting element. These are much bigger and more fundamental setting elements than mul origins, but for obvious reasons many people view them fairly queasily now. How much can you change them and still make Dark Sun feel Dark-Sunny?
Same with Dragonlance. I don't think too many people are arguing in favour of keeping gully dwarves as a race of cowardly filth-wallowing comic parodies of developmentally disabled people, for instance. So how and how much do you change them - or any other setting element - while making the whole thing still 'feel' Dragonlancey?
I could slot warforged into my Dragonlance quite easily for instance. They were an accidental creation of some gnome in Mt Nevermind who was trying to make something else. And he forgot to design an 'off' switch into his creation forge when he was tinkering, so new ones keep showing up. He views them as a bit of an embarrassing failure and pretends they don't exist, while some warforged try helplessly to earn their creator's respect and love, and some venture out as wide-eyed naifs to explore the wider world and maybe discover some god who'll care about them. Is it classic Dragonlance? Hell no. Is it Dragonlance-y and in the spirit of classic Dragonlance? I reckon it is, though I'm sure others would disagree.
It really is about where you draw your personal lines about what 'real' Dragonlance is. Something which goes too far for me might be fine for you. So there's going to be as many opinions as there are people., and everyone's going to have their little setting hot-buttons. Does a sufficently powerful bard or warlock or arcane trickster have to join one of the Orders of Sorcery? Can you have a raging kender or gnome barbarian while staying true to the 'classic' Dragonlance spirit, and if not, are you reintroducing class/race limits? If the appearance and clothing etc style of the Plains people are redesigned to make them less of an obvious Native American rip-off, is that ok?