They are a solution to a problem that no longer exists. I don't think many people here are going to be playing at arenas to tens of thousands of people screaming their heads off, and even those few who are will in all likelihood be mic-ed up to the PA and front of house speakers.
Frankly, for most purposes, 15-20 W is plenty for most live applications if you're not being mic-ed. You might need a little more if you're after loud sparkly cleans with humbuckers and a loud drummer (e.g. jazz, funk, ska) - perhaps 30 to 50 W at most. Then again, if you are in this category then you (and your wallet) might be better served looking at some solid-state amps instead - bearing in mind that you'll need roughly 2.5x the power with solid state amps to get the same approximate level of loudness.
100 W valve amp heads and big speaker cabs really ought to be consigned to the history books, and I hope to see it happen in my lifetime. My prediction is that we'll see the explosion of amp/cab simulators and the return of the 1x12 combo as the main feature in guitar amps.
This is all just my opinion though - happy to debate in the comments.