I don't think 100w amps are going away. Manufactures continue to release new models and they seem to be compensating for reduced volumes by engineering them sound better at lower volumes. Using Mesa as an example, they've recently released two 100w models (King Snake and JP2C). Both are multi-watt (10/60/100 and 60/100 respectively). Their most recent amps feature a mix of power scaling, pentode/triode, Class A/AB switching, built in attenuators and variac in an effort produce better sound at lower volumes.
Further, the 15/30w TransAtlantic was discontinued yet the Triple Rectifier is still in production. They've discontinued the 20/20 and 50/50 power amps while the 2:90 is still in production. The JP2C is designed to have more headroom than the Mark V. If they're still making it there must be a market.
If anything, I think small, low powered amps are eating away at the solid state and modelling markets. You go back 15 years ago and the average budget guitarist was using a Peavey Classic 30 or a Fender HRD with some distortion pedals. These days you can not only buy amps like a Tiny Terror, Nighttrain, but also the big name amps that everyone actually wants.... Marshall, 5150, Soldano, Mesa, etc.
Think for a moment.... When was the last time you saw a thread about a Line 6 amp? Crate? Marshall AVT?
Further, small amps continue to suffer the same problem they've always suffered.... they sound great, up to a point. And past that point you find yourself wishing you had more headroom. I think that's why on one end you're seeing manufacturers trying to make 30w amps sound better when turned up loud, whilst simultaneously making 100w amps that don't necessarily put out a full 100w.