I think that if mixing the races start right away, the respective riders would absolutely do well in their environments. Mixing MX and road racing would be silly. But TT and MotoGP are ~200 mph races. Mixed martial arts is more popular than single martial arts; I can't think of a single judo tournament that is done outside of the Olympics purely for entertainment. Not including such things as bumpy road surfaces, chance of going airborn at over 100 mph, shadows over the roads of the TT race, etc. the real difference between the TT and MotoGP are the riders. I think of the TT racers as more of self made street artists vs the MotoGP as hand picked and well groomed, well paid professionals.
Each of the two races seems to be able to claim as the fastest. I wouldn't mind seeing the question settled, realizing that chances are slim as there is no incentive for MotoGP to go to thr Isle. If the incentives and rewards are there, the differences would be reduced over time. I think that the TT racers would adapt easier to the safer track environment than vice versa. But the truth is, personalities and rivalries make for entertainment. I had no knowledge of times gone by, but the thought of Guy Martin mixing it up with Marc Marquez is something I'd stay up to watch.