I would trust Gabe's judgement 100%. Dude runs one of the most pro-consumer companies in history. In doing so, he has built a fiercely loyal fan base by simply being good to his customers; not trying to squeeze them for every single cent.

Gabe is a rarity; part of a bygone era of business owners and software engineers who truly care about their projects and want to build things that they themselves want to use and play. He's a smart man and Valve has been his baby since he left Microsoft. He'll make sure it's in good hands when and if retirement comes calling.

third option is he sets up some kind of foundation or trust arrangement and testaments his shares to that trust, which is then run by board of trustees as per trust charter. Usually meaning "well board of trustees is entrusted to see to the continued profitable management of the company by selecting suitable new management as comes necessary" combined with possible whatever extra instructions there is as to how to and underwhat principles the company is to be run.

Be it either private trust to benefit the descendants/described beneficiaries or a charitable trust with funds to be used for charitable causes.

Family trusts aren't that unheard of to exactly avoid the splintering of the ownership and thus risk take over bit by bit.