First, I must mention that WEB and Charlie have showed remarkable discipline in sticking to the foundational principles that BRK has been built upon, even in light of their extended ages.
Most mere mortals would have spent the past years, particularly given the huge cash pileups, making one last huge swan song bet.
The fact that the cash has been allowed to pile up, with a huge chance that WEB may not have the opportunity to ever put it to work while he is around, speaks to this discipline.

All that being said, it would be really irresponsible to leave the next generation of leadership with this massive outsized position that puts them in a no win situation just as they are coming to bat.
If the new leadership keep Apple and it goes down, they lose.
If they sell Apple and it goes up, they lose.
If they keep apple and it goes sideways for 10+ years ala Coke, they lose.

As others have mentioned, this crazy position was simply the result of a spectacular investment that was successful on a scale maybe never before seen in the financial markets.
Given the high PE, WEB seems to be doing the prudent thing by cashing in his chips.

And by the way...in the stock market, you have to feed the ducks while they are quacking.
The only thing more spectacular than BRK's buy was the incredible exit of this massive stake as prices continue to rise.
An almost impossible feat, actually most would say it IS impossible before WEB's miraculous trade.

And lastly...let's not forget that Apple is not some rare 1 of 1 Van Gogh painting that once sold, cannot be bought again.
Shares are openly available on the market at any time in the future if valuations become attractive.
This is not a burn the bridges decision.