A 2 wound model with a 5+ FNP is significantly more durable.
If you math it out, with two wounds you have a 50% chance to survive a wound from an over charged plasma after you have failed a save, as an example. Combine that with T5 negating the power of small arms.
At T5, 2 wounds and a 5+ FNP it would take 20 bolt rifle HITS with - 1 AP to remove a single plague marine, also.
Hence a 2 wound plague marine should be more costly than a Primaris marine, by a notable margin of 4-5 points at the least.
The difference is space marines in general just have many more options.
And even taking your example. 10 plague marines shooting back would be 20 shots (say 21 to make the maths easier), thats 14 hits, 7 wounds, so you kill 1 Intersessor. Thats assuming they dont have the cover save trait, which wouldnt even kill one.
Also, they can have far better board control with infilrating dreadnoughts, deepstrike denial, massive overwatch potential etc etc.
Their whole army, while maybe not as survivable are far more killy, and far more manourverable
I dont like it when people compare unit to unit, you should look at the codex as a whole.
Making plaguemarines 2W wound only be a start for making a deathguard codex not total garbage. I wound arguably say we are the weakest codex right now.
Just look at the PBC.
Its our only real long range shooting and it hits on 4+, Just how many armies is it useless against? Dark eldar, eldar, alpha legion, raven guard, admech, and even tyranids means its pretty much hitting anything on 5+ at a minimum, with a random number of shots. At that point you are paying over 130 pts for something that may as well not have any guns.
Knights just laugh at us. Assuming you have 3 PBC and roll 6s for all number of shots, thats 9 hits. Say you have arch contaminator and get 8 wounds, thats 4 getting though the invun, doing what 8 wounds on average. Over 400 pts, with exceptional rolling is doing 8 wounds to a knight with a 4++. In reality averages is meaning around 4-5 wounds. We have no answer to it other than grenades with bigious, VOTLW and Arch Contaminator...yea good luck getting that to go off against a compitent opponent.
The foul blightspawn can be amazing but is very expensive for a very random unit
Terminators just suck in any semi competitve game. Oh they are increadibly hard to kill....but with M4, good luck trying to kill aything
Our best unit right now is probably the daemon prince