There are way too many strikers who are good at stuffing the take down. Ground fighters without a striking game are at a huge disadvantage with a shirtless opponent in MMA. More than half of their submissions are taken away just from their opponent not wearing a gi.
Everything keeps evolving. Early UFC was dominated by submissions. Then came olympic style wrestlers with ground and pound. Strikers ruled for awhile who were good at avoiding take downs. Now it seems it's a 60/40 mix. By that I mean strikes are about 60% and submissions are about 40%. The problem with even good jui jitsu schools is that they really neglect take downs and take down defense. That's where guys with previous wrestling ability have a real advantage. You have to be well rounded today. A good striker with no take down defense is a sitting duck too.
