but whenever someone says "mass bends space time" its counteracted by "what's the mechanism" running the argument down a slippery slope
That's only because people incorrectly try to use the unknown nature of the mechanism for the UA as some type of problem to the theory. It defies logic to accept that one theory has an unknown mechanism, and then criticize another for not knowing their own.
It's not so much the fact that the unknown mechanism is a problem for the theory, but the following little conundrum:
You have an earth in which things thrown in the air fall back down to the earth. Thus you have one of two possibilities: Earth has its own gravitational field which causes the item to fall, or Earth is accelerating, creating an apparent gravitational field.
FE relies on the
assumption that this gravitational effect is an apparent one caused by acceleration. It then goes on to say, with no documented evidence, that there is something out there in space accelerating Earth through the cosmos. It seems, that with all of the assumptions and dismissals of other, more possible theories, by this point one should at least have some kind of an answer for what it is that is accelerating Earth. If one is going to believe in it (presumably someone does) then there comes a point where their belief should be confirmed not by a series of assumptions, but rather by evidence or perhaps a theoretical mechanism for what is causing the acceleration. This is where one begins to see some semblance of a necessity for a mechanism, since
nothing else indicates there is any acceleration taking place (like measurements that show a non-uniform gravitational field). At least if there were a mechanism devised one could perhaps begin to entertain the possibility.
At the same time, the theory of gravitation, though perhaps lacking an intimate understanding of
why or
how mass bends space, is a far more reasonable conclusion to draw than a universal accelerator. Suggesting gravity also does not even rely on knowledge of a mechanism, because there are enough experiments, studies, observations and examples of evidence that suggest it exists and acts in ways that we have predicted, and that it can be applied directly to Earth.