Yes a solid Spectre fix will obviously make designers rethink their microarch in some deep aspects. But somewhat good mitigations should be available as soon as the next chips, and off-by-default is completely utterly insane. The OS is not part of the platform (except in some special cases that can disable the mitigation for performance if they like), and the platform is supposed to be retro-compatible, maybe not perfectly but reasonably. Ok it has already been somewhat less true than before in the few recent year, but lets not encourage that behavior. So shipping new CPU that are broken by default but can be somewhat less broken as an opt-in is an attempt to mask the level of the fuck-up, or maybe to avoid the creation of a new stepping. We should not tolerate that from Intel, a stepping is expensive but they have the money to do it.
Did you mean to respond to someone else, maybe?
Of course, that'd probably need more complex logic to manage this new cache, which makes things more difficult. Then again, not sure how two caches interacts with potential speculation across multiple branches (does Intel even do that?)
One obvious approach would be to have two caches per core. Speculative execution would use a different cache than normal execution. If the speculative action is committed to, it swaps which cache is the normal one and which is the speculative one. Then you'd also need to flush the branch predictor on context changes. And a few other issues.
Nothing that's impossible to do, but it would require a huge amount of new design and a lot more silicon just to maintain the performance of current chips without mitigation.