Well they care about your [object] after they sell it to you if regulations make them. It's as simple as that. The fact is that if you buy a Google Pixel now,

Well they care about your [object] after they sell it to you if regulations make them. It's as simple as that. The fact is that if you buy a Google Pixel now, you already get 7 years of security updates with the Stock Android. And the people who care about GrapheneOS won't keep it 3 more years without the security updates (that only Google can provide for the proprietary bits). So I'm pretty sure that a GrapheneOS user will not keep their phone longer with GrapheneOS than they would with the Stock OS. But a GrapheneOS enthusiast will definitely buy a phone that supports GrapheneOS vs a phone that does not.