Should I be worried, or is this a cosmetic issue? floor in 1935 apt building
There are already so many elements to the original baseboards I think adding another one to the bottom, painted white, to cover the gap will look just fine. It will look different to the other walls but only minimally. fill the void with insulation first and lightly pin the new trim to the baseboard, not the floor.

It's a case of you'll know it's there, but it's unlikely anyone else would notice unless they have a particularly keen architectural eye!

My only caveat is to keep an eye on the problem just in case it is the outer wall bowing outwards. Don't forget the floor joists probably run into the wall so it doesn't want to be bowing outwards too much.