I understand that the best thing to do is to hold onto a good company forever. Let's say I need the money out in a decade's time and the company has grown, how do I monetize that paper gain without exiting the position?

Hello there! When you make an investment, I believe there should an investment objective in mind. One of the possible investment objectives could be to make down payment of a house where you invest in x company so that you can have y return in 5-10 years. When your investment objective has been met, I would say it could be time for you to sell. To be very honest, it is really difficult to know for sure one is a "good company" given that things might be different 10, 20 years down the road.

Hello! Really interesting question. I think the easiest thing to do will be sell small chunks of the company slowly. That will, you get to still participate in the company's growth for a significant period of time.

Holding on to a stock long term, yes, but not forever. As decades pass, norms/cultures/habits/technology will change and that will impact the industry and the markets. It is also likely more competitors will emerge as time goes.

Keep holding onto the stock until fundamentals change or better competitors available.