I understand why you emphasis on 'important cryptos', it is to eliminate scalability problem, but I don't get why you suggest this kind of 'pruning' for pegged currencies only and you don't go general.
I think here you misunderstood me - I mentioned pegged currencies only because of their possible function as a "gateway" to cryptocurrencies in general.
I mean it can be considered a kind of a solution to have crypto blockchains being somewhat clustered and for an arbitrary pair of cryptos to be exchanged both parties should find the shortest path between them and navigate by means of multiple swaps.
Here I see a problem: you would have to run several clients/wallets on your device (or one wallet-client supporting multiple chains) and if you are not using a "multiple full node" (that would consume lots of resources) then with every chain you "touch" while swapping the risk of attacks increases, and also you would have to pay transaction fees for every intermediate transaction. Via Lightning such a "swap path" may be possible off-chain, but here I really don't know the details and possible vulnerabilities.
Anyway, I think this "cluster" approach is what Blocknet and Supernet are trying to achieve, but their approach is still in alpha.