The Pixel 8 and Pixel 8 Pro will be receiving 7 years of OS and security updates. That official support is critical to GrapheneOS support, as without OEM support, nobody can ship complete security patches.
With this latest generation, Google is providing a phone that essentially gets the longest proper (read: full security patches, not incomplete or severely delayed) support for Android, rivaling that of the iPhone. I would go as far as to say that Google is outdoing Apple as well, because Apple does not commit to a date. They typically support devices for a long time, but there is never a guarantee like with Google here.
All this to say, it is extremely important to consider what the OEM support looks like and how much longer it is on for. A device past its EOL date is no longer secure. GrapheneOS does provide extended support releases for a while, but this is purely a harm reduction method and people are meant to swiftly transition to a supported device ASAP, as nobody can provide full security updates after that point (anyone claiming to do so is misleading their users).