so bitcoin is very hard to attack in this method. (51%)
but other cryptocurrencies are very vulnurable to such attacks.
so attacker can steal other currencies and then buy with them bitcoins...
how can this problem be solved?
If someone steals other crypto and then turns around and tries to buy Bitcoin with them, typically the market will correct itself so that the coins that were stolen are close to value-less and thus not worth trying to trade in the first place. Theres no way to solve the problem since there isnt really a problem to begin with, unless you try to make some equivalence between Bitcoin going up in value and stolen coins pushing the value up being a bad thing.