Hussar
Legend
His maths means nothing. It's a very specific scenario which is probably the least unfavourable to prove anything about these feats.
I have hundreds of dice rolls that prove that SS at the very least equates to roughly a 2:1 damage output. I find it more hilarious people are taking very specific white room maths over real play data.
It's really easy to test. You can even do it by yourself. Make some characters, make various encounters, use monsters from the MM and see what happens yourself.
The plural of anecdote is not data. Even if we take your rolls as gospel, it only proves that if I play the way you play and have a group that plays the way your group plays, I might have the same problem. In a group that does not constantly burn resources to make sure that the GWF/SS fighters are getting those buffs won't have a problem.
Fighter player to Cleric Player: Hey, on the first round of every combat, I want you to cast Bless on me so I can hit because this feat I took really reduces my hit chances.
Cleric Player: Umm, what? No. Look after yourself. I'm not burning concentration and my first round of combat every time, just so you can be a star. I think I'll be leading off with Banishment thank you very much.
Fighter Player: What? But, if you don't bless me, I can't hit. I'll be missing all the time. Why are you stepping on my fun?
Cleric Player: Why didn't you make a character that was self sufficient? It's better if I actually directly contribute to combat rather than Bless you and then make sure I never get hit and lose concentration. I'm a tank, I'm supposed to be on the front line. I didn't take this War Cleric so I can ride the bench and cheer lead while you kill everything.
Fighter Player: You suck.
I mean, I mentioned that a lot of this problem goes away if you actually follow the advice in the DMG and have 5-7 encounters per day, instead of one. The 15h level Battlemaster Fighter can burn his six superiority dice in one encounter, sure, but, he's got two more before a short rest where he's not hitting anything. The 15th level cleric only has 15 spells slots from levels 1-5 (I'm going to assume that he doesn't want to burn his 6-8th level spell slots on Bless). It's very unlikely he can afford to burn almost half his daily spells on Bless. And, I'm thinking it's a pretty rare player who would burn a 4th or 5th level slot on Bless in any case.
Part of the theory crafting that goes on here is how easy it is to rack up all these bonuses and gain advantage. Sure, that's true when you have 1-2 encounters per day (which, in other threads, Celtavian and I believe Dave Dash mentioned is how they play). However, when you put a bit of pressure on the party, forcing 5-7 encounters per day, pretty much all these issues go away.
So, for me, GWF/SS isn't an issue. The party simply burns through resources too quickly and would badly run out of gas in my style of play. By the 3rd encounter per short rest period and that 6th or 7th encounter per day, the party would get slaughtered if it used up so many resources so early in the day.