Pedantic
Legend
You're right about players for sure, that's always a problem, but my issue is with design. We have and might get more content, stuff for the existing rules/structure, but we're not going to get any development.But you don't necessarily need it either. If I wanted, I have enough material to run 4e, probably the least supported edition, for the rest of my life. The issue is not the material, but finding people willing to play it.
4e, for example, could quite reasonable use a reworking of monster scaling that incorporated later math into the initial setup. It could make a decision about whether the accuracy scaling feats should be included or not, it could build a bank of monster abilities by role to draw from in creating new monsters...there's a lot of design/dev space still there, based on the game's existence to draw from.
Unfortunately, editions in D&D before 5.5 have an established tradition of blowing up the world, and no one is going to go and iterated on an older design (and if they did, they'd alienate the limited set of people still producing content for it). The closest we came to that sort of iteration was Pathfinder 1, and it was under a lot of external pressures for both the timeline it needed to be delivered in, and how different it could afford to be.
It's honestly such a shame these are all editions of D&D, instead of different games that could reasonably produce their own new editions over time. Instead, as soon as we move on to the next version, the previous editions become frozen as lines of development, outside of whatever influence carried over to the current edition.