How usable is an old lithium battery with less capacity? - eviltoast

I have an older robot vacuum that has finally shown some age in its battery. The charger will charge for about 15 mins and then gets an error, but it’s enough to do a decent vacuuming of the room if I charge then vacuum, then repeat once more. I can’t leave it on the charger now due to the error repeating, so basically I run it dead until the next time.

So my question is, can I continue doing this since it works well enough, or is there potential problems/danger with the battery being at less capacity? I could buy a new battery, they aren’t terrible in price, but if it works and is safe, why not continue what I’m doing until it completely gives out?