Most of the improvements have been keeping it running in the face of being the place everyone stores and retrieves video, and that's a lot of invisible work (but is the real reason you don't really see working YouTube competitors; it's a technically challenging big-iron problem).
The knife in the back of Youtube competitors is that people ad-block because they don't like ads, not because they have some kind of moral compass. Youtube mostly tolerates ad-blocking, which pretty much cements it in place as the absolute best streaming service (no ads, no subscription, unlimited free consumption).
The result is that it's basically impossible to get off the ground to compete with yt, because early adopters are almost universally tech forward. And tech forward people don't pay for shit (except the 1-5% who do).