I get people not wanting to see Checo in an F1 car, but I think it's a good move. Cadillac isn't going to be aiming at the WCC from the get go. They want to hit the ground running and they need a decent driver with a lot of experience to steer the team before they set their sights on trophies. Ideally that driver should also know what it takes to be a top team because I doubt Cadillac is looking to be just a midfield team.
If not Checo then who? I'm sure they tried to get Bottas but he wasn't interested. There's also Zhou but he's a worse option than Checo. Any other driver would most likely have to bought out and even then, how many drivers would even be interested in spending their F1 years building up a team only to get the boot once the team sets higher goals?
Checo is not a bad pick, he'll do a few years in the Cadi and when the rookie next to him has matured they'll replace Checo with a new rookie.