I'm suspicious that people arguing that safety checks making things slow are basing that on the very stale now idea that CPU's work sequentially. Which isn't true for super scalar machines at all. And also forgetting compilers will optimize away a lot of them.
There is an asymmetrical risk here. On one hand the compile might emit checks that aren't needed. On the other that the programmer will fail to insert a check that is needed.
Both yours and parent arguments are strong and reasonable. Compared with other overhead that developers deliberately add to their programs, language safety checks are probably a drop in the bucket. We're in a world where developers think it's reasonable to run a chat app on top of an entire browser framework on top of the platform SDK on top of the OS. The runtime performance of checking an array's bounds are the least of our concerns.
There is an asymmetrical risk here. On one hand the compile might emit checks that aren't needed. On the other that the programmer will fail to insert a check that is needed.