Eh, if you plan on printing on billboards then sure larger sensors make sense but even today's fullframe stuff has more than enough resolution. The reason mirrorless are moving to new mounts is that the focal plane is different when you don't need a mirror box, hence why the adapters are bulky.
Heck, the A7 still uses a fullframe sensor so they're the exact same size as your 5D/6D/1D/etc.
That said the while the whole "feeds and speeds" thing is fun to argue, better photos come from good composition, framing and being willing to "zoom with your feet". I've seen stunning work from a 450D and seen people with more money than sense put out marginal work.