If the Seahawks are even remotely in contention, or even just fighting for a decent-ish record, we can't sit a starting QB to test Sam out. That's just... not really a respectable move in-season.
Also, we don't even really get much tangible benefit from it. If the team is doing badly, why would we throw an asset on a rookie contract out there to take meaningless snaps and risk injury? They'll have a decent idea of how he's developing by just having him in the building, and he's got a lot to rework.
If we even have to question whether Sam should be in late in the year, the answer is no. We'd be setting him up for failure and risking injury to him when we would much rather have him compete for the job next year than finish a lame duck year.
TL;DR: If we're hopelessly mid next year and Geno is playing poorly, let him take the lumps and then cut him. Finish out the year and protect Sam so he can compete for real in 2025.