web servers can refresh OCSP responses in the background and cache valid responses to add some tolerance against temporarily downtimes in the OCSP server.
It's really a chicken and egg problem. Browsers don't want to support must-staple because not enough servers use it. And servers don't use it, because browsers don't require it (or even implement it). And now CAs don't support it, because hardly anyone was using it.
Now if CAs started requiring must-staple, that might push it into widespread use. But that would cause a lot of disruption.
As for server implementaions. Most servers, sure it isn't that much harder to use `must-staple`, if you are already doing ocsp stapling. But most servers don't do the stapling at all, because there isn't a strong reason to, and you need to set up a system to periodically fetch and cache the OCSP signatures, and whatever system you use to terminate TLS needs to support it.