Cultural divide didn’t stop Islam from spreading to disparate cultures like those of South East Asia, Africa or Eastern Europe, so that cannot be the reason for the curtailment.
The original point still stands that Islam has no single mortal enemy. Plain reason is good enough to defeat it.
I wish you well, too, and hope you continue to ponder upon these questions just as surely as I will.
Which is a logical error. All cultures are not equivalent and therefore not equally susceptible to any given idea or system of thought. It may not be the case, but you have not demonstrated it not to be.
“Plain reason is good enough to defeat it.”
It would appear, at this point in time, that there is no “plain reason” in Europe.