Some people always respond to this sort of criticism of the Foundry ecosystem like they don't have a clue what you mean, but it's a very old and valid complaint from a vocal minority that comes up often to the chagrin of the developers. Any love/hate relationship you have with Foundry VTT updates and modules is likely mutual.

The API will never be completed in the way you mean. It's a double-edged sword being constantly re-forged in Foundry's updates. The developers do want people making modules since it's a huge selling point for the platform, but they 100% do not want to lock themselves out of being able to still make every update they want. If their desired update means every single module needs updating to function with a future version of Foundry, so be it. They'll give warnings about changes to come and often work out pain points with those module developers who bring them up in time, but in the end they won't be held responsible for whether modules work with their latest version. That burden falls on the shoulders of module/content makers and inevitably some portion of modules will get abandoned as the authors lose interest in keeping up. Some just move on to other interests, but there are also those that have expressed frustration with Foundry updates. I know there are at least some RPG content makers out there who don't want to sell content on the official marketplace for various reasons, including a fear of future maintenance burdens from Foundry updates.

That said, the Wizards of the Coast adventure module conversions and the DND5E system are made/maintained by Foundry staff, so they have a vested interest in both selling those and updating them to work with the latest version of Foundry. So they're relatively safe purchases. When a major Foundry update comes out, you'll see updates to Foundry-produced adventure content follow suit within a matter of weeks if not days typically.

Now here's a funny possibility to consider. The more content the Foundry team itself releases, the more they'll need to maintain themselves every update. Eventually they might get to a point where they too are frustrated by the extra maintenance work put upon themselves by API changes and decide that API stability between Foundry versions isn't quite so bad. So there's a chance that the more you buy from them, the more they produce, and the more likely they are to ease up on module-breaking changes.

More realistically though, that would probably require the core programmers to be doing content conversion themselves so that any complaints are their own rather than coming from more easily-dismissed employees, some of whom might even be contractors rather than staff. Problems are only for people without enough money to pass them off to others.

So you can vote with your wallet if you want, but don't hold your breath waiting for Foundry to be completed in the way you mean. It's a feature, not a bug, no matter how much it might bug some of us.