The west always assumed Russia would (attempt to) close the gap between Belarus and Kaliningrad at Poland's corner.

>The military powerhouse that is Kaliningrad

Kaliningrad actually does have a sizable (for Russia) military investment, which is why the west would expect them to defend it in that way.

I don't think Russia COULD do that anymore, unless it was entirely a Belarus operation. But if they succeed at rushing that gap, current war "meta" is extremely defensive-sided. Russia may not have a meaningful "fleet" but they certainly have enough working and pretty good submarines to make hell for anyone trying to then supply Lithuania.