That is what I have often been saying, it is those who already have money who will most likely make a lot of money from Bitcoin as well as they don't need to sell during bull runs. I really applaud people who knew it would be smart for them to sell at maybe 1k USD because they bought at 100 USD, but then held through all the roller coasters until it hit new heights like 30k, 40k and then the ATH with over 60k. But really, who does that? If someone isn't rich but firmly believes in the technology and still holds despite Bitcoin having its crashes, that's some mental strength there!
Regret is a difficult word here because on the one hand you are of course right that if you sold for a good reason and got some benefit out of it, it should all be good. But on the other hand I think some least amount of regret is unavoidable given that Bitcoin really has been a moonshot once in a lifetime investment that may never occur twice to most of us. I think this gives reason for some people to say to themselves that they do deeply wish they had not sold and instead endured some shortages for a bit longer. I know someone who owned 30,000 Bitcoin and sold them for $1 a piece to then buy a BMW I can tell you that he felt some regret when Bitcoin went 3 figures, then 4 figures and 5 figures eventually.
The problem is that I think too many people pull the hardcore trigger instead of selling only a fraction of their bag. Or they deviate from any upfront considered strategy, for reasons they consider to be right in that moment.
It's different from those people who check lottery numbers even when they didn't play in order to see whether they would have won if they played. That I think is a dumb move and I myself wouldn't do it, but if I did and I would have won, I would not feel regret. But when someone who was among the first miners and was passionate about the technology sells 30,000 Bitcoin to buy a BMW, I think there is a lot of reason to regret it later.
But when we talk about people who can't really afford tens of Bitcoin, but instead put some of their savings into Bitcoin and then sell at a maybe 500% profit, there is really no reason to regret it because from a rational and also risk aversion point of view, selling something at 500% profit would be the right thing to do in 99.9% of the time. Bitcoin just proved to be very different from what most of us have ever seen and probably will ever see.