That's a good point about centralization. For example Tor Browser forces updates (almost ridiculously often!), so indeed since Tor themselves would be implementing the pow code, there's a ton less lock in than (in the extreme) bitcoin. I do slightly wonder about e.g. random smaller apps (for example, on android? not sure) out there using fixed versions of tor (rather than tbb), i.e. they might become unusable for some applications with a pow change. I suppose it comes to the same thing - you *have* to update according to tor devs schedule (the whole auto-update thing in TBB is really dodgy now I think about it ... I guess it's related to it being Firefox ESR or something?).