I honestly don't see why anyone would be disappointed by the assortment of EXP blocks you get with R&D. There is tons of EXP blocks: MT bearings, ball joints, trailer hitches, trailer wheels, ramps, plasma furnace, etc. Some of them need to be graduated sure, but they aren't yet, and even if they were we'd still have a lot. To say there isn't any variety there just isn't really true. Once they start working on more complicated block additions, we will probably see more stuff added to EXP.
Sure, lots of blocks lately have been going directly to unstable, but that's because they don't need extra testing. At least, not as far as needing to find bugs, just balance, but you want to have as many testers for that as you can anyway. Keep in mind that they have been putting most of their time into multiplayer recently, so they have focused on simple, easy-to-add blocks. Those don't need to be put in closed(ish) testing.
Also, the whole point of R&D is for supporting the developers and getting to test blocks before they are added to the game. Making some blocks permanently EXP defeats that purpose, at least the second part. Anything that is in R&D will, and should, be added to the game at some point, unless they have been scrapped. MT blocks will go to corps eventually; they just haven't decided on which ones, and they aren't stable enough yet.
The exact opposite any other greedy devs would do. I know this isn't exactly a good thing, but how about have Payload Studios be bit more greedy and make more DLC for stuff you can't normally get? Because stuff like Canary status you have to earn which gives the devs a lot of hassle choosing who to add to Canary and they don't get much out of it. Nowadays, you have to EARN everything.
...no.
The whole point of Canaries is to have a small group of useful, trustworthy testers, and to encourage the community to work up to that status. Tying it to R&D defeats that purpose. It doesn't give people an incentive to post bug reports, it doesn't ensure that the players won't leak everything, and it doesn't keep it small for testing things that need to start with a small group.
The devs certainly get something out of it.