A large part of the reason those developers can exist independently is because they can target every DAW. So either you will have to hire them all or find new specialists, or develop that specialization yourself.
Even in the case of developers only targeting one DAW (Pro Tools), at least one company (AIR music) saw that it was worth the extra effort to release its products in other plugin formats like VST.
Honestly, I would not like to see new developers trying to oust the plugin standard. It's one of those quirks about music software that exists for very good reason.
I'm sorry, I don't get where this is coming from, the specialists don't need to be hired to write the same code again. The old code can just be re-used. If I could guess, I think this is stemming from a misunderstanding that "no plugins" means "no code re-use" which is an understandable misunderstanding (say that 5 times fast) but it isn't the case.
In my opinion, plugins only exist because it's convenient to package a synthesizer/reverb/whatever for users that way, but there is no reason that can't be supplanted with something that is more convenient. Of course if it's less convenient then that wouldn't be worth doing, if that is your concern then I agree with you there.
Not saying no code reuse. But if you're going to reuse code across multiple DAWs, then a plugin system is the established standard.
Do you think those existing developers are just going to drop their code into your environment and hit compile? How do you plan on getting all these musical components into your DAW? It doesn't work like that. Someone is going to have to write something, or you're going to be acquiring the rights to existing code somehow, which is still going to have to be ported to your environment.
Or you could skip all that and implement VST. There are even libraries that present a standard interface and output plugins in all the major formats, so you could target that instead. (I forget the name of the framework I'm thinking of but it was written by the Cockos guy and his DAW's ReaPlugs effects suite targets that library)
How would you motivate a synth/effects developer want to spend time on your project? Unless you hire them, and if you're going to ask them to "write reusable code," they're going to point to their existing portfolio of plugins.
It doesn't really matter who does the work, if you were writing a new DAW, you'd probably do it. But if you were working on an established & popular DAW, plugin developers could be convinced to do it if there was benefit to them (more optimizations, more features, more convenience, less bugs, etc).
I don't think it is currently popular for plugin developers to implement against VST themselves, the libraries you mention seem to be gaining a lot of traction, at least from my experience from trying to catalog open-source plugins on github.
Even in the case of developers only targeting one DAW (Pro Tools), at least one company (AIR music) saw that it was worth the extra effort to release its products in other plugin formats like VST.
Honestly, I would not like to see new developers trying to oust the plugin standard. It's one of those quirks about music software that exists for very good reason.