Evolution is path-dependent. Notice that mammals were comprehensively outcompeted by dinosaurs till asteroid removed them and gave mammals time and niches to develop in. If you recreated dinosaurs right now they would lose to mammals (for example to homo sapiens).
It's perfectly possible that mitochondria are the dinosaurs of "cell powerplants" that just haven't encountered the asteroid to let other (ultimately better) solutions develop.
Absolutely. It also means--however--that any niche where alternative did exist, when exposed to mitochondrial life, they lost.
Now that I think about it, it would be pretty funny if we're this universe's cheela [1], a freakishly overclocked biosphere that runs faster not because it had to but because it happened to.
[1] https://en.wikipedia.org/wiki/Dragon%27s_Egg