I haven't double checked your math but for the sake of argument I will accept it. Plague Marines remain quality over quantity. You can, if you wish, spam more basic Marines and there is a point at which the CSM become a better buy on paper. I don't think that translates to a better buy on the table. I base this merely on opinion and the games I've played so far. This is what I have gleaned (perhaps incorrectly) from my test games:
Plague Marines tend to be our only good move forward, take objective options. This is because they stand a better chance of surviving the ungodly amounts of shots and still being able to dish out some damage when they get there. The fact that FNP will allow them a roll on everything short of AP-2 or AP-1 firepower or weapons does make a difference. They are Fearless and cannot be broken. Basic CSM have an alarming tendency (in my opinion at least) of failing that roll and costing you an entire unit. If you factor in the cost of getting them Fearless or rerolls by other means, their cost goes up and the difference between quality versus quantity alters. Plague Marines allow you the option of a Blight Drones. I converted and built my own. These turn out to be rather cost effective, nasty buggers that come in flying on the 2+ turn with a Large Blast AP-3 template or a Flame Template. They essentially do the job of a Defiler at a fraction of the price w/the durability of a hard to hit flier. *Bear in mind I'm assuming you will be making your Plague Marines scoring via Typhus or an appropriate Lord.
Unless your taking template hits, more bodies at a cheaper cost for normal marines, or nurgle marines will do the same as your plagues, it might feel like your taking more pain with more models dieing but in terms of points coming off the table the change is not that big.
Ap1-2 taking away FNP was last edition, this edition its just instant death that negates it.
I had forgot about fearless, this is something you won't easily get on normal CSM since the icon is so expensive and it takes away your option for another mark like the nurgle one which is probably the most favorable one.
The main benefit of basic CSM appears to be more bodies for VOLUME of fire. This begs the question of whether or not we can get that better with Noise Marines, or if we can get close enough via the quality over quantity method. More and more this Edition is demonstrating that the gun is king and I am finding myself splitting the difference on bodies with some cheap allies (IG) or Cultists so the heavy work is done by Plague Marines and/or Noise Marines. What seems most effective is coming in with a damn Vendetta (along with my Blight Drone or Drones) to blow the crud out of things and have IG with lots of flamers disembark to hit also. What seems most important is critically mauling and quickly locking up the most effective and ever more numerous gun line units. The only more or less dedicated assault type unit I take is Raptors and even then I still like that they can shoot.
Noise marines do kick ass in a firefight where the enemy stays still, unfortunately since they have salvo weapons they pretty much don't do much outside 12+6" range since if you move into 24" firing range of a single enemy unit, they can just take a step back and now you can't fire at all. Salvo weapons have their weaknesses (or they move into firing range of 1 of your models, which you then have to remove first as its closest, and then you cannot fire the following turn. Its true nothing will beat a noise marine in a straight up fun fight (apart from perhaps GKs with their 7pts stormbolter henchmen) but they are rather hard to use
I realize I have gone all over the place but I do mean to tie this all together. A safe strategy is always going to be more bodies and more shots. Basic CSM can, via math, provide this. You sacrifice options, however, and I'm not sure that is the best way to build an all-comers (i.e. tournament competitive) list. It is my belief that you have to split the difference and get those options with the least loss of shots. Plague Marines provide this with their durability. Noise Marines provide this with their volume of shots. You offset this with the cheap chaff unit (you will have at least one) that is either allies or Cultists. This is because not ever unit in your army has to be great. It merely has to do the job assigned to it. You want to bring your most effective units against your opponent's worst ones. Screw a fair fight.

Plague Marines might not have a 4+ FNP anymore but they are still better overall despite the price.