Can you provide any arguments that JIT would in fact decrease security other than "Apple says so"?

Every major mobile and desktop OS other than iOS has supported it for over a decade. Apple is just using this as a fig leaf.

"Decreasing the security" is not binary thinking. It's just a fact today. Also, ability to run software doesn't make you less secure. I never saw any real proof of that. It's the opposite: Competition between different browsers forces them to increase the security, and it doesn't work for Safari on iOS.

I think a detached and distanced perspective must come to the conclusion that vendor lock-in isn't healthy. For security, performance or flexibility it tends to fall short sooner or later.

One could also talk about the relevance of a speculative attack that hasn't been abused for years. There can be multiple reasons for that, but we shouldn't just ignore the main design motivation of Apple here. That would be frivolous and that excludes serious security discussions.