Many people are going veg*n because its a 'new' fad and seems rebellious.
Some fads hold (think TV), others retreat to resurface generations later as a retro or another 'new' fad (think bellbottoms or baggy pants)
When its no longer rebellious and no longer worthy of TV time when a famous person talks about it the rate of increase will likely halt and then reverse. I dont expect for a moment that america will be 80% or even 40% vegan in the next hundred years.
Some people are doing it for health or genuine honest compassion, those are more likely to stick- but I fear they are the minority.
The decades will tell.