Problem with assessing Froome's riding is very simply that he has absolutely nothing to compare his Finestre performance to. The performance is as rare as it's stage design.
In that regard, it is perhaps smarter to use the Zoncolan stage for comparison, as that was his 2nd best day in the race, and it actually is comparable to other performances as it's pretty much your 40 minute all out W/kg drag race. And in that regard it's probably his best performance in a while too.
But I do think it's easy to misunderstand how riders decline. I think the top level on certain days stays much longer than the day in, day out consistency that is required to win GTs.
Froome still has incredible days of form, but his consistency is getting worse and I think it's hard to deny that. And it's much more likely that will be his undoing as a GT winner than not having his big surges of form.