Firstly, there's a very good chance driving without a speedometer is illegal in your area.
Secondly, anything that affects functional safety should itself be vetted fron a FuSa perspective. I guarantee you that most manufacturers treat the instrument cluster as a Functional Safety item. If CarPlay can't bring the system to a state where that analysis is affected (which seems unlikely here), there's no issue.
You shouldn't crash, but it's not unheard of for the speedometer to fail.
Certain early-2000s GM speedometers were prone to breaking. Either the servo motors straight-up stopped working, or the label started peeling up and jammed the needle. My brother had several trucks from that era with this problem.
