I get the idea you're describing and use it for various purposes. But I don't agree "there is no need to track the ownership or size for each object". You can offload some of that thinking to the arenas, the same way you'd do it with GC, sure. But you can't just ignore ownership - sure request context is owned by the arena - but do you need to copy the values you pass to logging? do you need to wait for log flush before destroying request context?

What about the more common cached data / process state?

Arenas simplify the processing just like GC but they're not magic and don't solve everything.