decent NUCs are much pricier than old discarded hardware
This can be true for some, but for a lot of labbers the increased energy cost of enterprise hardware will exceed the cost difference of the NUC over the expected life of the equipment. That doesn’t mean it’s an obvious choice to go the other way; it’s just something you should consider.
It’s just a technological step forward. Thread was designed from the ground up as an IPv6 protocol. Honestly, this kind of move is coming later than would have been ideal, given the massive growth in IoT devices.