The biggest risk to bitcoin comes from the US government, which may criminalize and ban it. In fact I am pretty sure they will do that once it gets into their radar.
However - and I am not joking when I say this - I CAN'T WAIT till it's illegal. Because once it is, the mainstream will find out about it.. and THAT's when more people will really start using it, and after a brief panic-induced selling from wimps, the price will probably skyrocket. Even though it wasn't money, that's essentially what happened with Napster, Bit Torrent, the whole p2p movement in general. Once they make it illegal, that's when people will start using it.
Napster original concept actually totally disappeared and so was defeaten, didnt get stronger like you're saying. Similar to many things like Kazaa, Limewire, etc. They never recovered significantly after all the legal actions taken against them.
Same with Bit Torrent, it's pretty weak compared to its top some years ago. Torrents are long replaced by news groups etc. Thus it has not gotten stronger.