Mervis has been quite good at scooping bad throws out of the dirt. That is probably the one defensive flaw that could kill a first baseman. He has also looked adequate on batted balls, admittedly, an extremely small sample size. But I agree that it will have to be his bat that keeps him here, and the biggest surprise so far is that his extremely small strikeout rate in all levels of the minors has totally reversed course in the majors. Hopefully, he will be able to adjust to what the pitchers are doing.
The biggest thing that he has going for him is that, unless the Cubs decide to move Bellinger to first base, there is no one on the team of in the minors that is likely to perform better in the short run.
Maybe Morel can give Mervis a few of his extra home runs.