They won't regret it because, as you said, no one could have predicted Bitcoin hitting $50k, and I bet not even Satoshi could have guessed it. So there is no regret, as long as they have made a profit from their bitcoin investment and are satisfied with their decision. I once sold bitcoin for 20k, and I made a huge profit, I don't regret it.
Predicting bitcoin to be 59k USD is one thing but holding on to that from a very low buying price, say three digit is another thing. Most people cant see it through unless they have the patience and a second source of income to keep all the bitcoin intact for years without selling a single satoshi.
Just keeping the cycle going on your portfolio is the real thing, with that you can forget about what is happening in the world and continue to increase the total amount of bitcoin in your hold.
Dont fall for the altcoins roaming around everywhere, they will only lead to small term gains and you will have to suffer the deeper losses when they drop.