Where did you come up with the idea that a person wants to be on their back specifically? As for thats the worst place to be in a street fight that is 100% true. However, are you going to be able to guarantee a person isn't going to end up there? Fact is, most fights do end up on the ground in a 1 on 1 situation unless the person gets a one shot KO in. Thats human nature.
As for Muay Thai, that requires a higher degree of atheism. A far longer learning curve for effectiveness, strength and general athleticism to developed. Bjj does not. It is gender friendly, does not require loads of strength, does not require athleticism, etc.
There is a reason why BJJ is being adopted by the military and not Thai.