But if Finland joins NATO, Sweden either must yield to RU or join NATO itself.
The key here is Sweden's the island of Gotland sitting at the middle of the Baltic sea.
https://www.google.com/maps/place/Gotland/@58.8670134,17.054...
In most scenarios it would serve as a most advantageous base for RU operations, including serving as an unsinkable airplane carrier / missile platform.
If SWE was the only non-NATO country in Northern Europe it would be quite open for an attack.
In case RU decided to invade Gotland from it's base in Kaliningrad, there is very little anyone could do to stop it.
The current conflict has taught that NATO forces will not engage with RU forces. If FI joins NATO, it effectively blocks FI from helping SWE to defend itself.
Basically, if FI joins NATO Gotland is open for picking for RU.