I'd happily embrace dracthyr gaining access to different classes either late into Dragonflight's lifetime or in the expansion afterward. I don't see any problem with that. Other races gaining access to evokers, on the other hand, I don't know if I'd really be all that into, unless it was done in a way that basically says "Lorewise, you're still actually a dracthyr, but for gameplay reasons, you are considered <race> instead, even though it's just your visage form." And even then, I dunno if it'd really sit too well.
I would be totally fine if hero classes like demon hunters and dracthyr retained some amount of exclusivity and aren't just spread around to all the races. Despite what Blizzard's sentiments might be, and despite how much freedom it would likely give players who want to play whatever they want as whoever they want, I don't really think it's necessary to give everything to every race, even with a little text box or a quest chain explaining how it totally absolutely makes sense. The basic, mundane classes, sure, flexibility where it's sensible to give it. I don't think hero classes should be all that flexible. Especially not one so intrinsically tied to its parent race as evokers.