Can anyone provide an argument for a necessary being?

[Ryantology]

If we can all agree that everything in the known universe does indeed have a beginning and an end, then we can rule out the idea that things can go on existing ad infinitum and also that the universe could have existed forever, can we not?

I don't think we do agree. There is too much ambiguity to say that the Big Bang is 'the' beginning rather than 'a' beginning.

Conversely, and nobody seems to want to tackle this, is the question of what would be a plausible alternative explanation for the origin of the universe without an agent that brings it into existence and sets it all into motion.

The necessary stuff, whatever it is, that is necessary for the universe to exist as it does may be eternal. No creator god is necessary, and because eternity lies at the root of both explanations, sticking a god in there just muddies the waters by adding a step in the explanatory process which, itself, cannot be explained.

Given what we know, there is no rational or logical necessity for a creator, so that hypothesis should be discarded unless future discoveries do indicate this necessity. I personally wouldn't hold my breath, because every such explanation ultimately seems to end in flaming tautological wreckage.