I do think they realize this and have been choosing certain villains to keep around long term to help build and develop them over time. Tbh, I think it’s part of their narrative formula or something in recent years. Of course in Legion we never directly fight Sargeras and he’s simply sealed away; can be used for another time. There’s also the fact we’ve known Odyn will eventually go bad since Legion and he’s been getting more development ever since.
BFA we have Xala’tath (though I don’t think she’ll be a villain and more anti-hero) as well as Azshara who escaped and is still out there somewhere. In Shadowlands as others have mentioned we have Sire Denathrius who escaped. I’m thinking in DF we’ll have such a villain as well, if I were to guess I’m thinking the frost sister is going to survive. Something about her tells me she’s more crafty than is let on, and she’ll end up having bigger future plans than even Iridikron. Give Blizzard some time, I’m sure one of the incarnates will survive to be carried into the future.