你好，Sam！ 1.可以去掉，去掉的句子表示对将来的推测，意思不会变， 我们会说：要是让他继续这样玩下去，没准儿就把沙发钻出个洞来，那怎么办？ 2.如果不去掉，句型是这样的：要是。。。就。。。了。（了在句末，表示对未来事情状态发生变化的推测） 我们会说：是让他继续这样玩下去，没准儿就把沙发钻个洞出来了，那怎么办？ eg: 要是天下雨，我明天就不回去了。 希望对你有所帮助,欢迎大家补充讨论~

In your this sentence '了' is used as a auxiliary word, when '了'used for this function, generally, used after the verb or adj. to indicate completion, at the same time ,which can also use in two different situations: 1.Used for the actions or changes that have occurred; Or 2.used for when you imagine that the actions or changes would be expected or assumed to happen , like in your this sentence, so you can't remove '了' , otherwise, there will be considerable ambiguity in your this speech . Because the person you mentioned wouldn't have done this thing in reality. But in verbal Chinese, you can remove the '了'，we know what you actually mean, if you take part in the HSK, you will lose your score because of grammatical problem.