I don't disagree at all, V8 was and still is revolutionary. However, the second order effect of V8 having captured such a large portion of the market is that developers will code/tweak/optimize until their app/library is suitably performant on V8 and then stop. No value judgement, it's totally rational but the effect is that now a lot of JS code has an implicit dependency on the quirks of V8 in order to be fast.

There's really no good solution to this because the kind of performance tuning we're talking about explicitly pierces veil the VM is supposed to provide. You're pretty much always going to target the internals of one particular engine. So what this means is that anyone working on a competing engine not only has to provide the same behavior but also the same performance profile because it's something that JS devs expect to be there now.