Have they ever discussed their reasons for switching to 5E?
EDIT: Nevermind, I found Mercer explaining why:
Pretty much what you’d expect: 5E is much smoother at the table, especially with 8 players and with Pathfinder’s copious modifiers. Also, D&D had more brand recognition and would attract more viewers.
I think two things:
-Critical Role would still have been very successful, but somewhat less so and would have taken longer to catch on, due to the exact reasons Mercer identifies. Probably would never have gotten quite as big as it is now without switching to D&D at some point. But still viable.
-Pathfinder would be in a much, much better position today and have many more players.