Marines have more than enough stuff that feels really difficult to remove. That includes their vehicles! Things like the Repulsor/Impulsor really don’t need a higher save tbh. The Repulsor is toughness 12, has a huge transport capacity, has a rule that lets you dodge a charge, and pumps out a ridiculous amount of shots (12 main cannon, 18 from one of the sidearms, and that’s before the 3rd weapon of your choice!!) versus the Toughness 11, d6+3 main shots, 2-6 sponson shots depending on which you take, and 2-3 from the pintle mounted gun of the Leman Russ, which has 13 wounds (vs the 16 of the Repulsor) and hits on 4+s instead of 3+s. The Russ is only 10 points cheaper.
That’s before accounting for the fact that tanks and artillery is pretty much the only thing you can rely on putting out damage with Guard, vs Marines where almost everything can throw out damage and hold things reliably
The same is true for Detachments, almost all of the Marine detachments are good! Lethal Hits sounds strong, but mathematically, let’s see…
10 Guardsmen shooting at 5 Intercessors. Let’s assume they’re either in rapid fire range or got the First Rank Fire order, so they’re putting out 20 shots, hitting on 4+s. On average, that’ll be 10 hits, of which 3 are lethal hits. Of those 10 hits, they’re wounding any Marine on a 5+, so of the 7 hits that aren’t lethal hits that’s 2 to 3 going through. That’s 5-6 saves of 3+, so on average against one guardsmen squad you’d be losing one marine, and maybe a wound on one other marine, for an entire unit that’s shooting. If your opponent is unlucky, you’ve an Apothecary attached to it and that one dead marine happily returns when it’s your turn! The Intercessors, meanwhile, will shoot 10 Bolt Rifle shots hitting on 3+s, wounding on 3+s, so that’d be 7 hits, of which you’re likely looking at 5-6 going through, which can only be saved on 6+, so there’s 5-6 dead an they need to do a battleshock test or be unable to claim objectives (their primary goal). Intercessors are only 20 pts more - the lethal hits on Guardsmen isn’t particularly that big of a deal because it only applies against other infantry, it’d be a different thing if it applied to other targets as well (like the Court of the Archon giving splinter rifles, too, lethal hits against everything. That’s a lot of S2 weapons damaging vehicles!)
Devs…