They will feel nothing. As at the time when bitcoin was around $10 or even less, bitcoin was still very volatile and at its early stage. Many of those early investors took the decisions they made because of the information and market sentiment available as at that time. They sold their coins and took the profits available then to sort out personal needs. Many of us here would have done same if we had some bitcoins then. Some of those early sellers might have reinvested, so there is no difference between those who sold their bitcoins when it was around $10 but reinvested and us who just started investing now.

Bitcoin is one asset that is worth waiting for, but if at any point you sell your bitcoins and took the profits available, there is no point living in regrets when the price appreciates further. Bitcoin has obviously exceeded our expectations countless times, no matter the price at which an investor buys bitcoin, he will still get profits if he's patient.