well I guess, if a transmissable variant we've learned to live with is stopping other variants from propagating then it's doing us a favour in the short run, until that evolutionary pressure dictates a mutation that thwarts us. For now, it's better to have something we can live alongside pretty normally than have an Epsilon variant that's evasive and sets us back to square 1.I don't think that adds up. Delta is as likely as any other variant to mutate into something nastier during any individual infection, and there's a lot more of it around precisely because it's so infections.
Delta first emerged late last year, and nothing has come along since, which has led to speculation that we're now at the point where even more tranmissible variants are increasingly less likely - if they were going to emerge they probably would have by now.
Evolutionary pressure is now I guess more on immune escape than transmissibility, as there are increasingly few people both covid and vaccine naive.