You are absolutely, right... your grumbling is valid. Those that really want old stuff printed in the 5E style, you are right to grumble that it isn't happening.Except it kinda is?
With the exception of mostly externally developed stuff (Wildemount) there are a finite number of projects that WOTC can/are willing to publish a year. For every Ravnica, you're delaying Planescape by 3-6 months, for example. And that's a best case scenario. Worst case, the legacy stuff gets pushed back so far they never get to it this edition because we need a Transformers book or whatever instead.
That's valid grumbling, imho.
The only downside is... I don't think WotC cares.
I suspect (in my opinion only) that WotC is of the opinion that those people who have played in the old settings KNOW what they love about those old settings, and will work really, really hard to run games "the right way" in those settings. And thus WotC feels like those DMs who do that? They don't NEED a new book to accomplish it. They are all experienced DMs who know what they like, know what they want, and if they wanted to run a game in Athas, they could do so right now using the Dark Sun materials they've gather from 2E, 3E, and 4E. Yes, the DMs would need to create certain rules to emulate older edition rules for metal weapon breakage, Defiling/Preserving etc... but really, for experienced DMs who know what they want, how hard is that to do? Either by themselves or to go googling to find other people who have already done that work?
Any DM who wants to run Dark Sun right now probably has several version of 5E psionics they could easily use or adapt for to run a game at this very moment. Now is it "official"? No. But who cares if it is? If you know what your Dark Sun game should be like... you can run one using 5E. You don't need WotC to hold your hand.
And this is why I think they have done all these other setting first (not including Eberron)... because there AREN'T previous versions of those settings in the game to use already. They are NEW settings for NEW players to find and enjoy. Us old folks who ran Greyhawk and Dragonlance and still own all that stuff printed in decades past? We know what they are. We already have what we need. So WotC doesn't need to cater to us anymore. They and TSR have done so for the past 40 years... they're now trying to help out the new folks coming in.
Yeah, it can be annoying, and yeah, we can grumble about it. Unfortunately, I don't think its going to change anything. WotC will continue to work on their own schedule and do it when they are ready.
(What I think really needs to happen is that there has to be a strong proponent of an old setting outside of the D&D design team who is willing to spearhead the project and keep pushing it. Like Keith Baker for Eberron, James Wyatt for the MtG settings, Matt Mercer for Exandria. Once they have someone who is over the moon for Dark Sun that they can trust to take the ball and run with it... maybe then it can move to the head of the pack rather than just be another "we'll get to it when we get to it" situation.)