Moving workloads in-house does happen. But, in general, you're right. It's hard to advocate for a near-term expensive (in time and money) and at least somewhat risky (expect some nights and weekends crises) migration for possibly some longer term cost benefit (assuming you've accounted for all the costs). Which BTW neither you nor your manager may still be around to take credit for. And also BTW is at least somewhat counter to what companies are doing in general, for better or worse, and which execs will probably rightly see as a potential distraction from whatever the company is trying to accomplish.

Frankly the whole discussion mostly highlights that these are things you need to think about upfront before you're fully committed.