I don't think they care because the devices in the future won't be subsidised anyway, they'll sell at a profit.

Plus if it's not ready then there's not much that can be done, don't want another RROD from rushed development.

Let me ask another dumb question.

If the next boxes are basically a PC in an Xbox shell, doesn't that already reduce any development time?

I do agree though that I'd much rather the handheld had been released a year before the Series X successor, I think it's a bit dumb to release them both at the same time because they're going to force people to choose one or the other with limited budgets, Idk. I'm already debating which I want first, something I didn't have to do with Series X vs S.

Funnily Jez leaking that Xbox has a handheld coming has made me completely uninterested in the OEM Xbox branded device releasing this year, and I bet they'll be others like me thinking "Why would I buy that instead of waiting for Xbox's own handheld?" Lol. Then Switch 2 is going to beat it into the ground but again I don't really care, I'm not getting it if Xbox is making a handheld, thanks Jez

As for your 2nd question, Idk, Lol. I would assume so. Might make it easier for xCloud upgrades too.

What would take more development time is if the devices are PC in nature, extensive testing needs to be done with different builds, and scalability of Xbox games. On top of the OS. If the Xbox OS is now gonna be open sourced, that also needs additional work and testing it works on a multitude of devices. 

Xbox hardware has been static up to this point, so this is new territory.