Games

First, if they already have a PC port your point is moot. They would still need to make a controller version for the Xbox and PS5, but the switch can use both.

Secondly, some games are simply not released on consoles because it simply doesn't translate well without a mouse, like RTS games, for example.

I want my Metroid Prime Hunters online (I still hop there with Wiimmfi)... Not everything must be Splatoon :/

Not exactly?
If they make it work nicely on surfaces that aren't perfectly straight, you could use it while playing on the couch.
Would be particularly amazing if games support switching between modes instantly, so you could for example be playing a game, open your inventory, slap your joycon on the couch to organize your stuff w/ mouse mode, close your inventory and pick it up resuming play in joycon mode 🤯

Turn based tactics or 4x games would absolutely benefit from both, and the touch screen. Being able to play Fire Emblem with any of the three depending on what's comfortable at the time would be wonderful.

People use mouse on Xbox. There will always be some

I doubt that no one will use it. If you make a good input method that works with a decent variety of games, some will switch to it. Perhaps Nintendo could even sell some sort of tray/mousepad that lets you use the mouse decently well while sitting on the couch. It seems odd, but I think it has potential.

From the article:

Though the livestream demonstrated the Joy-Con 2’s mouse function on flat surfaces, a press release shared by Nintendo to Business Wire confirms that sliding the Joy-Con 2 controllers over a pair of pants will also work.