Serbia is nowhere near the Baltics. Please consult a map.
So the Russians could not respond to a NATO move on Serbia by moving on a vulnerable NATO member. I suppose you would have seen no reason to enter North Africa in ‘42 as it was far from the actual target of Allied strategy. Yet 200k+ Wehrmacht troops were lost that could have played a role elsewhere.
The potential of an attack on a Baltic state could cause NATO to hold off a move on Bosnia.