To be fair, I never saw Sagan as a top tier sprinter, there have always been 4-5 better than him. He's always been more of an opportunist, best on long hard stages with a bit of up and down or uphill sprints.
As for the lack of monuments it's certainly not helping that there hasn't been a Roubaix for over 2 years, and he didn't have a great prep for MSR this year and did ok considering. MSR is the gaping hole on his CV obviously.
If anything it's the climbing and explosivity that has declined a bit. I honestly can't see him winning another Ronde now we are in the era of the Vans and Pidcock. And it felt telling that he didn't fancy a pop at the Giro stage Bettiol won the other day.
It's clear Bora has different priorities now so makes sense to go. TDE now would perhaps be in a bit similar place to Bora when he moved there. Will it be an issue he'll be targeting the same races as Turgis? Who looks a useful prospect. Do TDE have the strength in depth he needs to support him on the hilly intermediate stages?
Could be an inspired move, but could also be a car crash.