there is no relationship between these stable coins and cryptocurrencies. so this question doesn't make much sense to me. the fact that these coins using another chain (like Tether using bitcoin blockchain) doesn't mean the main chain is relying on them!

stable coins are only satisfying a need that exists right now. the need to first transfer money between exchanges without needing banks and second the need for something stable in market that is extremely volatile specially altcoin market.
if some day that purpose could be achieved with banks (like having faster bank transactions with lower fees and little to no KYC) then stable coins would die overnight.