An epilogue to this story:
Today I went to a different dentist.
I chose a completely different dentist, totally unaffiliated with the last one, in a different town (actually much closer to where I live now).
I got a second opinion.
I had this new dentist tell me I absolutely did NOT need to come in every 3 months, that I absolutely did NOT need the more expensive "periodontal maintenance" appointments, and that there was no basis for any of that. I got a clean bill of health, dentally speaking, that I've got no cavities, no periodontal disease, and no notable dental problems.
So, I'm going to the new dentist.
You have not been vindicated. Yet.
Because what the new dentist told you agrees with what you were already wanted to believe, it may seen like this is correct. That's called confirmation bias and happens with literally everyone. Traditionally, going against any innate bias is hard, you want to ignore or discredit conflicting points of view.
But what you have is two different professionals with two different opinions. Nothing says which of them (if either) is correct. If you want that, find a tiebreaker. Know for sure.
Also, does the new dentist have access to your dental history (the old should be willing to fax (*cough*) them over). Specifically about you having gum disease before?
Yes, I have been vindicated.
You're placing a LOT of trust in that first dentist's opinion, that just happened to mean they'd be able to bill me for a LOT more in dental work.
That first dentist also said I could *never* get regular cleanings again, that if they even tried to bill for it, the insurance wouldn't let them, that they had no choice but to bill for the more expensive "periodontal maintenance" visits.
When I was doing my intake with the new dentist, I told them all about my prior dentist, their claims I had gum disease, their claims I was ineligible for regular cleanings and insurance would never pay for it for me again. . .they told me that was baloney. They even had to call to verify my coverage, and while they had them on the line called to verify that (I asked them to verify, since they were very insistent about it). . .the insurance company said no, that's nonsense. A regular cleaning is a lower-cost dental code, they'd much rather pay for that than the higher-cost "periodontal maintenance" code.
So, that's one thing the old dental office told me that's been outright proven to be false. . .and it just happened to be something that also meant they could bill me for a LOT more money.
They don't have the actual records, they didn't request them, I did verbally tell both the front office and the dentist herself what they'd said to me. The front office said they were outright lying to me about the billing practices at the other place. . .and the dentist said she saw no sign I had gum disease or ever had gum disease, and even if I did, saying I'd have to come in every 3 months for the rest of my life was absurd. She said if I ever did have gum disease, it had completely healed (and as such was probably pretty minor to begin with) and there was no indication I'd ever had it. If I hadn't told her about it, she'd never have known otherwise.
Let's remove the medical aspect from this. So, if I have car trouble, I take the car to a mechanic, and they say I need $2500 in automotive work. . .and I take it to a different mechanic and they say it's a simple $150 fix. . .I should get a third opinion because I have two separate opinions? In the real world, that means that it's a $150 problem that a mechanic tried to soak you for a $2500 bill for.
Same thing, Dentists assume people trust them, so if they say you need a lot of dental work and expensive procedures, people trust them. . .taking it to a second opinion that sees absolutely no need for all that work and procedures means a lot. There's a reason the term "second opinion" is used to describe getting confirmation on a questionable or dubious diagnosis or suggested course of action, not "third opinion".

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.