As a mobile game developer who primarily works on graphics and has to worry about a range of hardware across iOS and Android, this thread is going to make me want to gouge out my eyes. This is the opinion of an expert in the field and it can't be dismissed with throwaway lines about them already having to do that for PC.
One of the primary benefits of consoles as a concept is that they are a single hardware target which you can focus on. It is far easier to get close to the maximum of what a single piece of hardware is capable of than it is to get close to the maximum of what several pieces of hardware are capable of. Compare a console and an equivalently powerful PC and the console version will usually be significantly better in visuals, performance or both. PC gaming makes up for this with substantially increased power on all fronts.
Adding more consoles inevitably makes things trickier for developers, and the S will be no different. Getting the most out of the S and the most out of the X will be a significant challenge: in almost every case you will have a less optimal experience on one or both. This is already the reality of multi-platform development, but clearly the addition of the Series S exacerbates that. Overall the S should expand the Xbox user base and maybe even the console user base, and that should make up for it because developers will sell more. Doesn't change that it's going to be a pain.
I think it's worth remembering that not that long ago Phil Spencer had a lot of people here convinced that releasing on the Xbox One wouldn't hold stuff back before the rhetoric was shot to pieces by the Halo Infinite showcase.
Let me guess, this thread won't be closed?
Why should it be?