Re: Downey's last?
The thing about reboots in the MCU is that they're pretty damn unnecessary. Ever.
Sony and Fox like rebooting things because they only have access to so many characters, so once they feel things getting stale, they feel it's time to start fresh.
If things get stale for a character in the MCU, you can just focus on films featuring other characters and maybe just have the "stale" character make some cameos. And if you want to make another film with said character, a recast doesn't necessarily constitute a reboot (we've already seen as such with Rhodey and Bruce Banner).
I'd rather the MCU just continuously work forward without reboots. In fact, with the MCU, I don't think they are even possible. If you reboot one character, you have to reboot the entire MCU. Recasts will inevitably happen, but reboots I don't think ever will.