Now that the discussion on that seems to be winding down, let me just add a few pointers for future reference (when posters read this thread):
This thread is actually only about a fairly trivial point, namely when to use the -5+10 part. A lot of words, but still, not terribly difficult math.
The real issue is how often you can expect to get to turn on the feat.
The feat's defenders seem to labor under the impression this doesn't happen too often. And that the corner cases where you do use it (and use it correctly) still doesn't provide the expected benefit (because the monster died, letting hp go to waste, for example).
They do not realize that this mechanism (the -5+10) part is the centrepiece of every minmaxer's build. It's a source of much more damage than anything else available to a martial character.
And, the cutoff point (the AC number where the -5+10 part provides a statistical benefit and so makes you turn it on) is rather higher - given good play - than many of those defenders realize.
Or, sadly, want to realize. Because they insist each game element should be evaluated in isolation, which is a naive stance every minmaxer immediately moves on from.
Something as innocous as Bless giving +1d4 is, by itself, a cool buff. But combined with GWM it almost singlehandedly wrecks the intended balance of the feat.
So is Bless overpowered, broken or unbalanced. Not especially. Is GWM overpowered, broken or unbalanced? By itself, again, no not especially so.
But Bless does become unbalanced when its bonus is applied to reducing or negating the to-hit penalty of GWM. Yes!
Does this now mean Bless is at fault? No. It would be absurd to try to remove all the ways you can get bonuses to your to-hit. Remove Bless and the GWM player will simply use the Bard dice. Or his own Reckless advantage. Or any other effect that reduces or negates the -5 part so that the AC cutoff point is lowered to make the feat an unprecedented source of extra damage, so much so that the game is hurt by it. You simply can't get that kind of damage unless you choose a weapon compatible with either GWM or Sharpshooter, which is bad for choice.
Back to Bless. Why target Bless when the real culprit is the feat. If you remove Bless or any single other source of to hit bonuses, the problem remains.
But if you remove the feat, there is no longer anything to base your damage build upon. It is the feat that is the source, not those other things that I call the enablers. They just enable the feat, they're not broken in their own regard.
Instead of bending backwards not to have to accept the feat is outright abusable, why not remove the feat (or replace the -5+10 part with a half-feat such as +1 Str etc)?
I still cannot understand why some people can't (or I'm again sorry to say: won't) see this easy connection. I don't have a problem internalizing that WotC made a mistake. So what they didn't think things through. It's not the first time the minmaxers find stuff that the design team missed. It's not the end of the world. I still love 5th edition. I just would have loved it more if it didn't provide such low-hanging fruit for the minmaxers.
Instead of squabbling whether the feat really can be abused (it can) why not present an united front towards MMearls and the other designers, making them understand the feat will need an overhaul sooner or later.