No you are wrong. The VRR is different because it does it on the fly so you do not need to lock the Hz. Steam Deck does not have VRR, so it has to lock, but can lock to any Hz you want. Is not limited to 90Hz.

Lets say you have a game that fluctuates from 30fps to 50fps. On the Deck your best option to to lock at 30Hz (or an integer multiple). This looses the extra fps but this is not that bad, you get it on battery life. On ROG Ally that has VRR you can live it uncapped and get all the extra fps which is nice (and also have less than an hour battery life)

There is no benefit but since Valve went for this screen that has this capability why artificially limit you for the refresh rate you choose? Mind you that you can ran on 60Hz if you want to, so there is no downside.

not everyone needs to switch, if you don’t change your needs then you are perfectly fine with what you have.

Steam Deck (both LCD and OLED) is better. Your PC is aging to the point of being beaten by a handheld.