I'm not sure which other Republican candidates could really step up. Aside from Trump there's DeSantis, who seems to be imploding and I think hates Trump anyway, Chris Christie (who called him a 'One Man Crime Wave' lol) and most of the others I haven't even heard of before.
But there is also Mike Pence who is truly a wild card - there's a reasonable chance Trump literally tried to have him killed and yet he still defends him for some reason - so if anyone is bonkers enough to make it to the White House and then set Trump loose, it'd probably be him.