secrec
That's precisely what I commented on here under my recent thread on size and weight (I find larger phones, so pixels from the 6/6a upwards completely impractical). Not everybody was happy about it.
But I do get the impression that Graphene is a more solid system in general than DivestOS, although the latter does seem to be the second-best option.
https://eylenburg.github.io/android_comparison.htm
I'd say it's feasible that a Pixel 5 with Graphene now almost 1 year after it's last security update is more resistant to certain threats (but perhaps not all) than the latest updated version of DivestOS. Proof? None.
And then there is practicality and ease of use, where Graphene is probably a winner. It also has a stable channel, while DivestOS on Pixel 5 is only "Reported working" if I well remember. App compatibility is also key, and DivestOS is more of a gamble in this respect (or seems to be anyway).
As an added extra, Pixels from series 6 upwards seem to be plagued by issues that weren't perhaps that common with earlier phones, and Android 15 also seems to be more buggy than 14. That's purely based on comments here and on other sites like reddit.
Comparing GOS with normal stock android, which is often weakened by manufacturers and not hardened, doesn't really make sense at all. There will be many aspects where even an outdated version of GOS will be better(e.g. proper sandboxing). As for more recent vulnerabilities... those are a risk. But they are a risk with stock Android as well, because other manufacturers do not issue weekly updates like Google (not even monthly), and you may only get the latest Android version with a huge delay.
Another aspect I'm acutely aware of is that new phones have got so big that some people (like me) are turning away from them. GSM Arena lists almost a 1000 new Android phones released in 2023-2024, and less than 2% of them are below the weight of 170g, which is not that light at all. The Pixel 5 is the last lighter Pixel at 151g and may be one of the best options around if you need a smaller phone that's also not riddled with security and privacy issues. I may get myself one...
I'm now using a P4a 5G which is slightly larger and so a bit of a compromise for me, but it has a jack output, and is otherwise technically very similar to the P5. I bought mine refurbished at a good price when it still had support (which ended a few months before the P5's, so I did not get one last major update). I never had any issues with it. The 6a is the next smallest Pixel that still has support, but it's significantly bulkier, and apparently also has various issues like dropped calls with some carriers and an easily cracking screen. Also issues with Android 15.