What decentralised network has more hashing power than btc ? what are you talking about don't make sense.
If BTC is the only major decentralized network it doesn't mean that it could economically back fiat currencies.
how weak ? what do you mean that ?
From the national point of view rates of their currency should be stable and at the same time this currency should have liquidity. For now BTC have none of those features and nothing tells us that it would ever get them. This is why BTC is too weak.