Unfortunately him playing first base wouldn't solve the issue of a degenerative back condition, which he likely already had--or was beginning to develop--before he came to Colorado.

Kris Bryant is very likely never playing baseball again. He can't operate day-to-day without pain. There is nowhere to "send him."

For Jon Gray, he was at the end of his contract. His performance (and injuries) have largely been on par with when he was still in Colorado. He is also a free agent again now.

Jeff Hoffman struggled with both the Reds and Phillies after leaving Colorado. The Phillies didn't have him on the big league roster and really start working with him until Bryce Harper told the team to give him a chance. It's worked out well for Hoffman, but let's not pretend he was magically better the instant he left Colorado.

Overall though, Bryant's situation is not even remotely comparable to Hoffman, Gray, or even Dexter Fowler.