1) "homegrown systems are expensive to engineer, but cheap to run" misses the most important part - how much does it cost to maintain, and how much of a RISK is it?
Homegrown probably means you're depending on tribal knowledge from a few core people who if they leave you're hosed. That's a risk. You also have to train EVERYONE you hire to learn the homegrown systems, instead of hiring people with externally transferrable skills that can come in and hit the ground running.
2) all of the on-prem folks had to additionally take on that responsibility
Most transitions/migrations always end up with a period where you have both systems running. It's not surprising that at first everyone has additional responsibilities. It sounds like the "on-prem folks" either didn't have the skills to work in the cloud or refused to learn, and needed external hires. Well, that's one way to put themselves out of a job, and that would be the next step once the homegrown system is deprecated.
Obviously there's a reason why "not invented here" syndrome exists. There's a reason "build vs buy" is a complex discussion because of more than just buy costs. People always prefer the home-grown thing, DIY. Engineers especially. On this site, particularly. And, also, plenty of successful businesses exist by cutting out layers and going on a shallower stack ("your margin is my opportunity").
But at the end of the day, for the vast majority of businesses, companies, and software shops, managing their own in-house infrastructure is a worse decision than paying a cloud vendor.