In December of 2007 Berkshire was trading at 2 times book. I didn't sell any shares. Had I done so with conviction, I would be considerably richer today. (As it was, when prices got halved, I turned over every rock for capital to buy more.)
From that 2007 peak, shares have gained about 400%, (CAGR a bit under 10%), while the shares I bought November 2008 have gained 720%. It works out fine to fall in love with a wonderful business and never sell but it might not optimize returns. It certainly is less work, and if it is a truly great company, less actual risk to simply hold, regardless of valuation. You might not be correct determining fair value.
I'm now retired and this again changes the calculus for me. I'm more interested in retaining what I have. Berkshire is as close to fairly valued at the moment as it has been since 2007 and I've sold some in retirement accounts. At some time, Fairfax will likely become fully or even over valued. I'll sell when I think so- it has been a hell of a run.
If it keeps being run as it is today, never selling a share should also work out fine. It is tricky to maximize returns- things can become dear and stay dear for a long time. You have to be right in your assessment that you will be able to re-enter at a lower level, and that's beyond a lot of peoples pay grade.