ASIC's will [probably] be the end of the road, but definitely not the first generation.
They'll just follow some moore law pattern, just like CPU's do.
I think that the hashrate will grow for the next 100 years and it'll still be profitable.
I think there's enough inertia in the current migration to ASICs to drive difficulty past 100M this year. So much of the cost of hardware is not ASIC, so I don't think Moore's law will help for a while.
I guess it depends on the value of the currency.
Am I wrong, but doesn't mining end around 2040?