Does that timeframe line up with Ek getting injured?
It very well may. If you're asking him to produce the same way with significantly worse linemates, you're asking him to be a Kaprizov-level player (or better still, as we'll see). Being able to produce 1v3 or in extremely limited offensive zone-time are exceedingly rare skills to possess. When he has linemates that are anywhere near his skill level, he has driven play. If your suggestion is to trade Boldy for a Kaprizov level player, you're going to be disappointed by reality.
For reference:
Mojo - Hartman - Boldy has CF% = 38,7%
Mojo - JEE - Boldy = 59.2%
Mojo - Gaudreau - Boldy = 47.7%.
Note: I took CF% because it was right there on Frozen Tools, and no other reason.
It seems fairly obvious to me that the center quality of a line matters a lot, even for a lines most skilled guys.
Looking at Kaprizov:
Zucc - Rossi - Kap CF% = 51.9%
Boldy - JEE - Kap = 57.7%
Boldy - Rossi - Kap = 56.8%
Looking at Kaprizov's most common linemates, we can see that Zuccarello and Rossi are worse than Boldy and JEE, at least when playing with Kaprizov. If we ignored the last line of data, it would appear that JEE is the driver for all of this, but then we see he's not on the bottom line of data, and the CF% is nearly as good without him as with him.
tl;dr If any of this proves anything, it's that Kaprizov is the best player on the team by a good margin, and that good players get better results when they can hold the offensive zone. JEE is great at holding it, Rossi is pretty good at holding it, Kaprizov is awesome at it, Boldy is good at it, Zuccarello is sort of bad at it, Hartman is terrible at it, Gaudreau is not as bad as Hartman at it but still not great. Boldy won't produce as well if his line can't hold the zone, just like Kaprizov doesn't.