At 70.7 years old and a owner of Berk since Jan 10, 1975 I'd be willing make a bet that Costco the business may do quite ok but at some point there will be stock price that will be a "high" that won't be attained again for at least 20 years. Valuation changes, slightly more competition, slightly less capable or successful management...or simply business/social developments overall can make this happen. But to me the valuation of Costco comes very close to assuring this will take place...even if the business stays sound.

As to Berkshire I'd be willing to make a bet that without a home run elephant sized investment that the runway will be very much like the historical runway but at a generally - and again slightly - lower overall valuation and a progressively progressively slower annual gain going forward from here.

Of course I'm not selling, but being realistic is one of the many helpful models that you can stay grounded as to your actions.