Soon" would be in the next 1-3 years or so.
One of the biggest dangers for BTC is something more useful coming along and investors switching - a process which could happen extremely fast, once started.Only as long as the coins bring similiar advantages for the users, like BTC and LTC. No huge differences there, so I can definitely see 2 to say,5 crytos co-exist there. But imagine something came along that blew BTC out of the water - some coin with all the advantages of BTC, but much more improved than BTC! That would be a completely different ballgame game there.
So to all the tech savvy people in here - what advantages could other cryptocurrencies bring that will give them a crucial edge over BTC, and how fast could something like that come along?