They don't need to trade him right now, but they may have to trade someone with salary eventually, and I think the argument is that you can probably get more in a deal for him now than when he is 28-29.
Personally, given how Kakko and Lafreniere have been slow starters, I wouldn't be opposed to extending Buchnevich on a pretty long deal. I think NYR will be able to trade Trouba and Kreider in 3 and a half years which will open up a lot of space. Also, there's a near zero chance that Strome doesn't depart, either by trade or UFA eventually.
In 2 years time you can potentially have 3 lines with scoring wingers. Panarin, Kreider, Laf on the left, Buchnevich, Kakko, Kravtsov on the right. That's solid balance. Combine it with the top blueliner prospects we have in the pipeline and, minus center, that's a complete roster.
Who slots in at 3C behind Zibanejad and Chytil is the interesting Q going forward, and honestly I think the appropriate answer might be a Vinny Propal-esque bargain bin veteran signing. There are going to be a lot of players bought out with the flat cap, so opportunities will arise.
In two years if NYR don't make any major trades I could see the following lineup
Panarin-Chytil-Kravtsov
Kreider-Zibanejad-Kakko
Lafreniere-?-Buchnevich
4th line
Miller-Trouba
Lindgren-Fox
Robertson-Lundkvist/Schneider
You've got 8 scoring forwards on that roster, so four per PP unit. Two PP defensemen (Fox, Lundkvst/Trouba) If Robertson and the RD even turn out to be halfway decent, that has a potential to be a top 5 defense in the league.
Panarin, Kreider, Trouba, Zibby, Buch are the veteran leadership.
Then, when Trouba and Kreider's NMC's end in 2024, you move them out to make space for long term extensions for the new core, who would be ready for leadership roles. Potentially Buchnevich gets moved around then as well. Reassess where Zibanejad is into his new contract.
I'm making a lot of assumptions with this roster, but you can see how the moving parts could align for a very long competitive window (3-4 years with Kreider-Trouba-Panarin, the following 6 years with the new core.)