For every commentary you have which states Paul was afraid he would be "unfit to enter into heaven", there is commentary which states the verse relates to losing the incorruptible crown for which Paul was contending ... just as shown in the context.
1 Cor. 9:22. To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means
save some.
23. I do all things for the sake of the
gospel, so that I may become a fellow
partaker of it.
24. Do you not know that those who run in a race all run, but
only one receives the prize? Run in such a way that you may win.
25. Everyone who competes in the games exercises self- control in all things. They then
do it to receive a perishable wreath, but we an imperishable.
26. Therefore I run in such a way, as not without aim; I box in such a way, as not beating the air;
27. but I discipline my body and make it my slave, so that, after I have preached to others, I myself will not be disqualified.
What did Paul
preach? He
preached the
gospel—“ the power of God for
salvation to everyone who believes, to the Jew first and also to the Greek.” (Romans 1:16). In 1 Cor. 9:22 he writes that he became “all things to all men, so that [he] may by all means
save some.” In verse 23, he writes that he does “all things for the sake of the
gospel, so that [he] may become a fellow
partaker of it.” He then presents to his readers analogies from sports, and sums up his discourse by writing that he disciplines his body and makes it his slave, so that, after he has
preached to others, he will not be disqualified.
Disqualified from what—from a foot race? No, disqualified from
salvation.