It's a real bummer that MS isn't going to make exclusive XSX games in the first year, but I'm not worried about the GPU, I'm worried about the CPU and the SSD. If the previous generation is supported, it means that the CPU won't be utilized much more than making things that the Jaguar is already able to do but in higher fidelity while the SSD will be hardly used except for shorter loading times and streaming higher-quality assets because the old HDD will still need things like loading masking.
The only way I see it not hurting XSX games is if the developer's vision never needed an SSD and a powerful CPU or if the game is developed in a vacuum for the XSX while a third party like Iron Galaxy will have to deal with the burden of the port, cutting whatever they have to in order to get it running. I guess it's OK if, for instance, a game on the One has to be broken into levels size chunks and had pre-rendered cutscenes in order to mask loading while the XSX version is one seamless campaign without any loading times.
On the GPU front, I'm not worried much, anything can be downgraded, turned off or made using a different method. If they are willing to go low enough and invest the effort, anything is possible. CPU and SSD use on the other hand? Well, it's just one year, as long as Forza Horizon 5 is made exclusively to the XSX I'm OK with it.
BTW, one of the biggest bummers is that RT can't be used to save production time because they still need to put in the effort in things like cube maps for the Xbox One version.