That's what shoe molding is for

Can you flip the baseboard (top to bottom) to see if it has any crown?

Depending on the length of the piece of base, you might be able to flex it down to conform to the floor, then nail it off.

Or scribe it.

Personally, I would not accept a deviation like that in trim work. It's either full contact between the base and the flooring or a consistent 1/16th or so gap. If the baseboard can't conform to the floor, it gets scribed.

Then there's always the idea of if the floor is dipping, why it's dipping. Break out the bottle jacks!

"Then there's always the idea of if the floor is dipping, why it's dipping. Break out the bottle jacks!"

Old houses have settled.

If you want a new house, build one.

I second brickeyee's sense... old houses settle, subfloor isn't always even, etc.

To answer your question -- is this your home? Will YOU be bothered by looking at this everyday? I think that should answer your question.