If your loved one is in a facility make sure their teeth are being brushed. I made a bad assumption that this was happening. After 2 years my mother told me "her tooth was hurting". I took her to a dentist and I can't remember what the outcome was, since this was 20 years ago, but I remember the dentist coming out and just saying, "I'm so sorry."
Apparently her teeth were a mess. I said, "The nursing home was supposed to be helping her with brushing her teeth."
The dentist said, "Well, they're not."
If I could do it over I would have looked in her mouth occasionally when I visited her 1-2x a week and discussed with the staff if she was brushing or allowing them to help. She had dementia, so I can see how she might have been uncooperative.
A visit to a dentist every 3-6 months would also help.
A toothache can make life miserable, and I think back and wonder how long my mother might have had her toothache.
Some dental problems will happen just because we outlive our bodies and the treatments would be too challenging (or expensive!) at that advanced age. Some problems occur because dementia patients aren't cooperative and the facilities can't force them to brush their teeth. In other cases it's because they're on Medicaid and not many dentists accept that coverage (in my personal experience). My MIL is on Medicaid in a LTC facility. They have a mobile dentist come in once a year to do cleanings and check-ups. She is in a wheelchair from back problems and has a difficult time being leaned back enough for the dentist to work in her mouth, but they do what they can. And, like you said, some facilities don't ensure the staff is doing what they are supposed to do.
My 92-yr old mother has been borderline obsessive about her oral hygiene all her life. As a result she still has most of her teeth and they are in good condition because she brushes 2x a day and flosses, sees the dentist regularly. I replaced a bad cap with an implant, which will last forever, and am glad I did it. I've decided to avoid needing to repair/replace further crowns and caps, or need dentures, by getting implants as I think it is worth the investment and is one less thing to create a problem in my future, should I live so long.
If a LO is on Medicaid, a dentist should be brought in every six months to check on patients. You can always take a LO to their own dentist.