As a youngster who has had to pick up a saw and chisel to repair an old chair, I think you're looking back with rose tinted glasses a bit. I've never been astonished by legacy code, as much as bemused for the lack of object orientation, data normalization, etc. The technology changes of course, but there are advancements made in craftsmanship/techniques.
There's some 'cleverness' lost probably, in the new ways vs the old. I've seen some pretty novel approaches to what should be simple tasks in legacy code. And I do recognize that for what it is. But as impressive as that cleverness can be, I think it's one of the reasons as to why a lot of legacy code persists and nobody wants to touch it. Among many other reasons.
Technically speaking, if there's any decay in your teeth, that's a cavity. The deeper it is, the faster it'll get worse. When you want to get that filling though, is a judgement call that a lot of dentists make for you sadly. Is it best to go ahead and get those fillings now? Probably, as far as your dental health is concerned. Can it be delayed though and done later? Almost certainly. Just a bigger filling later. Or maybe a crown if it gets too far gone.
Also, how fast your decay progresses depends on a variety of factors. For some people, it won't be long before those questionable cavities become serious problems. For others, it can be many years if they decay much further at all even. I think it's largely genetic, but also depends on oral care, diet habits, etc.
So, no, I don't think most dentists are just seeing dollar signs and putting fillings onto teeth without cavities. Its not outright fraud or malpractice. But some dentists are sympathetic to your financial situation while others think dental health should be a priority above everything, no matter the cost.
So it's a mix I think, between dentists' values/philosophies about dental care, and the lack of science about how fast tooth decay can vary from person to person (and an inability to measure that).
There's some 'cleverness' lost probably, in the new ways vs the old. I've seen some pretty novel approaches to what should be simple tasks in legacy code. And I do recognize that for what it is. But as impressive as that cleverness can be, I think it's one of the reasons as to why a lot of legacy code persists and nobody wants to touch it. Among many other reasons.