By the context OP is given I would say partially yes.
1. OP is specially talking about the Xbox platform, not in terms of gain from the game
2. Microsoft Windows is not Xbox platform
3. There are multiple reason why Xbox is failing, releasing game day one PC is only one of them
4. If Nintendo somehow release their first party game day one on PC, they will sold more game and make more money, but that doesn't mean Switch as a platform is unaffected.
5. Releasing the game on PC day one will of course NOT make the game worse, but it will make Xbox as a platform less unique.
6. Day one release is pro consumer, however it negatively impact Xbox as a platform in someway, because it give one less reason to purchase that platform
However that is far from the only problem that is causing Xbox to fail as a platform, I personally would say the factor that impact them the most is unable to release games that is appealing enough to capture consumer's desire to purchase their platform.
In fact if you take a different perspective: The reason they are releasing games day one on PC or on Game pass is because of their anticipation that the game will not create big enough impact for consumer to purchase their platform, thus in this situation the best way is to gain as much as possible from that game instead of building the platform, however this mindset is ultimately their downfall, its a vicious cycle. If you take this perspective you could see that they are not willing to sacrifice temporary gain for the goal of rebuild their Xbox platform, basically this is their way of giving up on the platform without telling you they are.
Another perspective: They are just using their console business as a stepping stone for their subscription and cloud service based business model, its not crazy to guess that they might just quit their console business in the future, either perspective its clear that Xbox as a platform is not their current focus.