Before Russia withdrew troops towards Ukraine the border to Finland was well-staffed. Actually capturing Murmansk is countered by Russia’s nuclear doctrine (first strike on attack of Russia’s nuclear capability), though I’m not sure taking out only Murmansk would be enough to trigger that, and of course strategic ambiguity is desirable there from Russia’s POV.

But it’s a wide-open flank even if Finland were to immediately turn south, not towards Murmansk. All of Russia’s flanks are wide open, right now, towards NATO, towards the Stans, towards China. If Russia truly feared an attack from NATO it would keep the NATO borders manned, not put those forces into Ukraine, and if it was worried about China well there’s no helping it if China wants to take a bite out of Manchuria there really isn’t anything Russia can do because going nuclear isn’t worth it. Maybe nuke Vladivostok out of spite or something but they’re not going to risk the imperial core.

Leave it to the Kremlin to be dealt an excellent deck and squander every opportunity of greatness on pointless cruelty for “surely that will make them respect us now” sake. Pathetic.