Just an update -- sorry to take so long -- but I did get a second opinion, and the second dentist said that none of that work was necessary. The "holes" in my teeth, she said, were normal wear, and that it would do n harm to leave them be. My gums were okay too, and she said that as long as I took proper care of my teeth, brushing and flossing, I should be okay. My usual place (which I won't name) was evidently scamming me. I won't be going back to them again.

I've had a lot of dental work, and I've never heard this one before.

I and family members havd visited dishonest dentists before. This sounds very suspect and to the financial interest of said dentist. I would seek a second opinion.

Yeah, they should have done x-rays again if they want to fill any cavities, that sounds shady. I just went and they took X-rays and showed me some tiny holes that they said they want to fill, but they also said that my teeth are pretty strong and that we could even wait a year to fill them since I don't have dental insurance.
Your experience reminds me of a chiropractor I saw once who I think upon seeing I was a VA patient just saw dollar signs and said that I needed to come back every week. I didn't agree with her so ended up going to a different chiropractor who doesn't even charge me if I don't need to be adjusted. He's amazing.

I have never heard of periodontal care requiring lifetime treatment. I would get a second opinion.

You may need a second opinion. I have a new dentist and they use a deep cleaning (takes a while, and they numb your mouth) process. It is questionable that they didn't take any xrays before and why. You may have the holes, most likely. I don't know about a 4 time a year for life treatment - research I suppose.