If the common hardware customers already have fails when you attempt to use a newer codec (but works fine otherwise), that can't be said to be a mainstream codec yet.
Your particular model is not representative of "the common hardware" as a whole. And it can be mainstream even when a significant fraction of devices can't use it. AV1 is a third of Netflix streams and most YouTube streams, that's more than enough to qualify as mainstream.
If the common hardware customers already have fails when you attempt to use a newer codec (but works fine otherwise), that can't be said to be a mainstream codec yet.
Your particular model is not representative of "the common hardware" as a whole. And it can be mainstream even when a significant fraction of devices can't use it. AV1 is a third of Netflix streams and most YouTube streams, that's more than enough to qualify as mainstream.
It's on most of YouTube streams because Google forces it on devices that lack hardware support, even if it breaks the functionality of the device.
Mainstream support doesn't have to be forced.