Based on the
data, it doesn't seem that play-driving metrics and scoring rate metrics decline equally. It doesn't agree with common perception, but Kreider's main value comes in his defensive impact at 5v5 and his scoring at 5v4. Kreider is an
elite winger because of his ability to drive transition, puck possession, and raise scoring rates on the powerplay. If he's not scoring at 34, there's reason to believe he'll still be very effective in these areas which are, after all, his greatest strengths as a player.
With Panarin and Kreider, the Rangers have arguably 2 of the top 10-15 LWs in the league on their roster. Especially since after Zibanejad we're looking at near-teenagers with very thin resumes in being impact NHL players as our centers (not to mention having our hopes at RW tied up in younger, if more promising, players who have yet to play a game), having two first line wingers who are elite at driving play and getting their linemates the puck in all three zones would be invaluable not just to competing now, but to aiding these kids in their development and putting them in positions to succeed.
I agree with
@Machinehead that Kreider gets seriously underrated. If the Rangers are serious about icing a competent team this year, losing Kreider without a clear replacement in the system could be disastrous. As much as you have to worry about what years 5-7 of his next contract could look like, you have to consider how those first 1-4 years might go without him and what it will mean for years 5-7 if those first ones are terrible.