I'm far from saying athleticism is all it takes.
In fact, i think athleticism as in being insanely strong or having crazy cardio is horrendously overrated.
The reason i'm saying football players have insane talent for MMA, is because their talent in coordination, football players at that level *all* have brains which are insanely good at processing fast movements, meaning things like catching a ball, playing a ball accurately over a distance with your feet when the ball is mid-air, or - in the case of MMA - strike with astounding accuracy; it's the exact type of talent guys like Silva, Izzy and Conor have, their sense of distance and their timing is on a level which isn't due to learning, but largely due to having a talent for it that's off the charts and so do basically all soccer players at that level.
As for the rest; Van Dijk might not be crazy strong in the upper body, but one can work on that and in terms of cardio, he'd very fast be the best in whatever division he'd play in.
In terms of cardiovascular fitness, soccer players are the best athletes on the planet in sports which are not about running itself.