If a cryptocurrency becomes too big to fail, then it seems inevitable that nation-states and large corporations will mine it at a loss.
This is why cryptos as currently implemented will always be fringe and used for the most part illicitly, because as soon they gain a wide enough currency the participants in the system will switch to a more efficient, i.e. centralized system.
If enough people use them that a government or large corp would care, those "enough people" would form a new government or become the customers a new corp, that facilitates their transactions in a more efficient way.
...and a touch ironic that you seem to be pining for TBTF cryptos when Satoshi himself railed against the TBTF system.
(I don't consider that scenario likely, just possible)
Currency exchange is a solved problem with costs and overhead that is very likely much less than going into the "securing the blockchain" business, even as a side gig.
How much "securing" would they need to do anyway? Just a little to help them sleep at night?
Not to pick on you particularly, but in general there is so much mushiness and hand waving in the coin space about what exactly is the value proposition other than to the person selling you the coin at a profit.