I've got plenty of high quality (Maxell, TDK) cassettes which sound about as good as they did 40 years ago. I've got some poorer quality cassettes (Realistic) which have suffered brittleness, de-lamination, and mechanical breakdown.

the better brands were put together with screws, so if you lose that contact pad (and you need it there to play back accurately), you can pull out the parts and either glue them back together or you can replace them.

It is possible to splice cassette tapes (once you open them up to retrieve the ends) but finding good splice kits is getting tougher.

If you have an album you care about, chances are good you can find a CD of it. If you have a mix tape or family voice tape you care about, consider digitizing it or having it digitized and 're-released' on CD.