There's no doubt Bob Foster was a great fighter, probably one of the best LHW's ever, but he was a LHW. Let's not forget that in 1968, Ali would've been in his Absolute Prime, and this is what makes him so great. for all the things Ali achieved, he did them either side of what would've been the 3 finest years of his career!
Yeah, Ali maybe did try to chip Foster for a bit more dough, but the reality is because it was Ali that brings the money, not Foster.
And anyway, how would it have even been possible for them to fight in 1968?