Why doesn't gravity weaken?
And why doesn't the magical force that is pulling everything away from everything else at an accelerating speed weakening?
Ok, both of you. The OP is about 'why UA doesn't weaken'. Why are you attempting to turn the argument around just because your own theories don't make sense?
By the way, reversing the argument doesn't even make sense. The force of gravity is dependent on the amount of mass a body has. In gravitation, there is no reason for gravity to weaken. It's so fallacious to assume that the weakness of UA pointed out in the OP and following comments also apply to gravitation.
In this universe, everything decays. Yet the force of gravity has remained unchanged since it was discovered. Why does it not make sense to question that (beyond that it's the accepted dogma, of course)?
UA is a fundamental force of the universe in FET, just as gravity is in RET. It would be fatuous to the extreme to try to argue that UA must weaken at some point while gravity is allowed to stay at full strength forever. I'm not "reversing" any argument; I'm using an example from your own theory to show that such a question is nonsensical.
And meanwhile, as Pongo points out, the expansion of the universe continues to expand in RET billions of years after its formation at an accelerating rate. Duh, why hasn't it started to slow down? As usual, REers are just being silly, accusing FET of being guilty of traits their own theory shares.
Okay, I think may be able to offer something here. I believe the intended question is: what is the formulation of UA? You see, as you move farther away, the effect of gravity becomes less -- it's strength "decays." But despite an accelerating Earth for many, many years, the UA effect is unchanged. So this begs the question: can we present a model for UA, that has a rigorous formulation, explains what we observe, and is self-consistent.
This is a very natural question to demand of any theory, and we should demand it of our own. In comparison, dark energy already HAS this formulation in Einstein's Field Equations. So if we wish to keep things fair between FET and RET, then we are behind.
Also please be careful about these RET claims you make. The increased rate of expansion of the Universe is a very recent phenomena, and has NOT been going on for "billions" of years. Also, it is erroneous to state that everything in the Universe decays. This is not true. You could construct a specific statement, about entropy, for example, and make a true statement about "decay" in a manner of speaking.
Effectively, if UA is a fundamental force, just as gravity, then it deserves the same formal description that gravity enjoys.