If Evans is signed to e.g. 4 years at 4.25 million per, then there is a significant chance that he will be untradable Gallagher-style in years 3 and 4. He's playing this season like an above-average third line center, but he's really an above average 4th-line center.
Current forwards making between 4-4.25M/yr
Ryan Hartman - 30 years old, 15pts in 43 games
David Perron - 36 years old, 0pts, in 10 games
Patrick Kane - 36 years old, 30pts in 42 games
Alex Iafallo - 31 years old, 15pts in 49 games
Dawson Mercer - 23 years old, 23 pts in 50 games
Nino Niederreiter - 32 years old, 26pts in 49 games
Viktor Arvidsson - 31 years old, 16pts in 33 games
Robby Fabrri - 29 years old, 12pts in 35 games
Ross Colton - 28 years old, 16pts in 32 games
Marcus Foligno - 33 years old, 15pts in 48 games
Mats Zuccarello - 37 years old, 33pts in 35 games
Carter Verhaeghe - 29 years old, 34pts in 39 games
Trevor Moore - 29 years old, 20pts in 36 games
Jason Dickinson - 29 years old, 14pts in 47 games
Looking at this list, all of these deals were signed in a salary cap between 80-88M...the cap is projected to be between 92.5-100M over the next 4-5 years.
How exactly would signing Jake Evans to a 4.25M/yr deal make him "untradeable", assuming you even want to trade him in the first place?
Signing him to two years (for which he would then be overpaid) limits the risk. At the end of the two years if Evans is really this Eller/Staal level third line center he'll get a great contract. If not then the Habs move on to one of Hage/Newhook/Beck/Kappanen/Xhekaj/etc.
Why would he sign for 2 years unless you're willing to make his salary well worth it?
Again, i'm not quite sure what the beef is with a deal in the 4M range, people aren't updating their software when it comes to contracts. You're no longer dealing in a salary cap in the 80Ms.