It all depends how you define "cost." It's not a debate between Rossi and nothing, it's a debate between Rossi and other available centers. Any UFA is going to make far more money for far more years, whether that's a younger mega player like Sam Bennett or an aging vet who you might sign for a just a couple of years to improve the teams short-term fortune. A "better" RFA is going to take a bigger offer sheet (especially if his team is more likely to want to keep him). And a player who's already under contract will cost more in trade.
The likelihood of an offer sheet is still almost zero in any case. Only one other player besides the two guys last year has changed teams on one since 2021, and nobody did between 2008 and 2021. And the guys the Blues signed were cheap (a 2nd for one and a 3rd for the other).
Until the draft happens and we see what the Flyers do with their other two picks it's hard to really guess what might happen anyway. Certainly if they trade one of them into 2026 it suggests they want to leave an offer sheet option open. But Rossi is still more likely to be acquired via trade, whichever team gets him.
But BE, when AF says "regress' he doesn't mean "decline." He is arguing his numbers suggest he's not as talented as he might seem and aren't sustainable, not that he's going to become a worse player in the short-term. Of course at his age he could also still become a better player, or have better numbers on a better line (like with Matvei Michkov, for instance).