Imagine using passer rating instead of quarterback rating to evaluate QBs.
I don't think it's terrible to use Passer Rating as a metric, but overreliance on it as an 'overall' assessment can be an issue. Passer Rating gives you a decent idea of basic QB stats (Completions, Yards, TDs, INTs) and is a valid point of comparison on that specific metric. (And QBR being a 'proprietary' stat is its own issue).
That said, it ignores a lot of things that go into what a team wants from a QB and in Russ's case ignores some important negative aspects that a coach like Payton might justifiably want to distance himself from.
While Russ was better in Passer Rating than Geno, so fairly better than him in the areas measured by that metric this year, he was bottom half of the league in Success Rate, QBR, and ANY/A and Geno was top half in all of those.
QBR and ANY/A are pretty easily explained: Both of those stats take into account getting sacked and passer rating doesn't. Russ was a sack taking machine. His offensive line was rated top 10 in both Pass Block and Run Block Win Rate, yet Russ was still pressured at the same rate as Geno who had a bottom 10 OL because Russ had the 3rd longest time to throw in the entire league. And while Geno was great at avoiding sacks this year despite pressure, Russ wasn't.
Success Rate is that area that Seahawks fans should be quite familiar with from Russ's last couple of years here: He's a boom or bust dude. He's got great Passer Rating because when he hits, he hits. He makes miracles. But in between his miracles is a string of 3 and outs. His average dropback is less likely to be successful than Geno's average dropback and that leads to an offense that has no rhythm.
Russ is a very specific type of QB. As a backup, our offensive philosophy would have to change tremendously from Geno (assuming Geno is our starter next year, though that's not guaranteed) and even though Russ was better in one specific measure of QB performance, he was worse than Geno in most others--so he'd be unlikely to get the starting gig. It makes no sense to have a backup QB who is older than our starter and who would require a completely different offensive philosophy if he were to come into the game.
I'd much rather we focus on drafting a backup QB who could theoretically become our QBOTF than bring Russ back, even at league minimum.