I would take a screwdriver and gently tap on the rusted areas to see if they're sound. If the screwdriver make a hole - you're on borrowed time. Be aware that inside is going to be similar to outside, if not worse. The good news I think is you can still see the bottom track so if the wall is solid enough the wall is being contained in the track.
I can relate to your issue as we have a sloping back yard that we made an area flat for the pool. It can collect water after a rain and the water eventually goes away. Our new pool is in the same area as our old pool so I suspect eventually it will have the same issue of rust as our old pool. When the liner ripped on our old pool I was going to put a new liner in until I saw all the rust on the inside walls and the bottom track was gone. I would have been ok with the inside wall rust if it passed the screwdriver test but no bottom track was the deal breaker. If the walls passed the test I was planning on using rust converter and painting the areas with outdoor paint to cover those areas. We got 19-20 years from the pool so I can't complain too much! Some people do a french drain around their pool to divert water away, I never bothered to do that.
If the rusted areas pass the screwdriver test use rust converter and spray paint the areas. I would test the areas every so often to make sure they're still solid