Undefined behavior means that certain expressions, like 1/0 in arithemtic, have no meaning; i.e. they are grammatically correct but nonsensical. For something to be a language, it is necessary for it to be able to state things that do have a meaning. But if some of its sentences have no meaning, it does not necessarily impact those that do. You don't need to "remove" anything. By default, sentences people say have meanings but not all of them do. To understand the sentence "I like pizza" you don't need to "remove" any undefinedness, just as you don't need to "remove" undefinedness to determine 2 + 3 = 5 even though some arithmetical sentences are meaningless, like 1/0 < 5.

It's like asking which is "worse", saying "England is a country in Africa" (false) or "Love ate a circle" (meaningless)? Both are "bad" in their own way.

But undefined behavior can certainly be inconvenient, because it is insufficient to determine whether a sentence is "well-formed", i.e. grammatically correct, in order for it to be meaningful, as some grammatically correct sentences are nonsensical. But it's certainly not catastrophic, because mathematics has a lot of undefined sentences, and we still use it successfully.