The human player plays like he plays because he is lazy. Attacking 20 targhets at the same time requires a lot of work. And i am talking AI splitting like in small fleets of 6-7k. Just enough to kill space stations fast. Then once that is done regroup. While the doom stack can destroy only one objective at time.
That's the same reason why in EU 3 in multiplayer no one ever used doomstacks and won a game. If the AI splits only in two it makes no sense. I am talking to split a 150k fleet in 15 fleets attacking 15 space ports at the same time. Then you can tell me how good your doomstack of 150k will be at dealing with that.
And let's say that every one of this 15 fleets has enough troops to conquer a planet. You lose 15 planets in less than 6 months. What good is your doomstack then if you cannot even pay for it?
As Sun Tzu writes, "He who defends everything, defends nothing." As long as I stop enough of those initial attacks that the warscore isn't high enough to force me to admit total defeat, I can turn around and win it. Every. Single. Time.
Again (and this is really where your argument falls apart, IMO), naval capacity penalties are a joke. With enough EC, enough minerals, and someone willing to trade my minerals for more EC, I can fund a war for
decades even if my EC deficit is triple digits (I know this, because I have done it.)
Though I really only need to fund it for *a* decade. Why? because that's when the Length of War penalty reaches 0 and the AI starts to make demands based on what it can get (or offer surrender based on what it has to give up) until that point, the AI will only make demands if the warscore reaches 100%, or their total is enough to meet all their demands (which, considering that they always try to go for everything they can, is basically the same thing) This isn't something you can really change, either, because there is no easy way program the AI to tell the difference between "you're one step away from total victory" and "your gains are built on a house of cards, make your demands now."
The long-term loss isn't much to worry about either. Yes I might have sunk thousands of minerals in each of those stations, but the actual effect to my economy would probably be 45 (15x3) EC, even less food, some research, and... that's it? If the AI is taking the planets in under 6 months, they aren't doing a lot of bombing either, so those worlds should be pretty much intact, majority of which not losing anything and maybe *one* where a pop died.
So I just hunt down the fleets, one at a time, warscore from those victories and my ever-increasing fleet advantage (you are not rebuilding a 150K fleet in a decade) sustaining the gap from the AI's total victory. If they go together right after the initial push, that just makes this part quicker. Maybe I take back a couple worlds; Not so much as an reclamation, but again to keep from loss.
Once that's done, then I split
my fleet to hunt down and destroy every one of their stations so they can't rebuild, leaving something behind to bomb each of their worlds so they can't rebuild their stations, then I take my semi-sweet time to reclaim all my worlds and begin taking theirs until
I win.
Splitting your fleet
before you decimate the enemy fleet is a *bad* idea, because there currently no realistic way to do significant enough damage to their economy to cripple their fleet in that way in time to matter
at all, much less in the war in question.
PS. Doomstacking is less about "keeping everything in a single fleet" as it is "keeping everything close enough that they can immediately respond to threats on the other" first is silly in some cases, second will let you see victory 99% of the time no matter how much the other size tries to out-micro you. There is currently no reason to ever spread out your fleet beyond that point unless you are the only thing left in space.
PPS. You know why blockades no longer generate warscore? because people were splitting their fleet 20 ways at the start of the war, positioning them so they were in range of all the enemy worlds, and then declaring war and wining within the first two months based on the warscore from blockades alone.