I think for any army you create it is possible to imagine it losing to any number of things, unless it was created using something very overpowered/broken. For most people, this probably involves imagining the new army against the types of things they have recently played against, which makes sense, but also probably overestimates what the opponent is capable of in most instances. In terms of behemoth, I think there are some problems similar to what you describe, but I think the points made about RPS are double-edged:

You describe how having cheap champions is a near-mandatory advantage against behemoth, but also say that having more points in minion line is a disadvantage. For this reason, the real behemoth RPS is probably closer to dove than drake, because a heavy costed minion line will force you into having more of what you describe as the counter to behemoth.

I also cannot accept 'boring kingmove' as valid negative, so among these things the 'spam behemoth until they run out of champions' type scenario is largest problem to me, and has been proven to be hazardous in the past, but honestly I don't think that strategy is viable anymore. If it were a problem, that may be a time where a tier shift could be used for something other than buff/nerf, simply to phase out low tier behemoth and raise the champion cost threshold of what can be traded into it, which would also raise the tier of lifestones involved in attemting to spam base tier behemoths.

There is also an alternative of: behemoth cannot be targeted by units below 5 value
This is a sidegrade that allows, immunity to smallest champions, most minions, etc. but which would allow the heaviest minions a chance to fight against the minotaur combo, and allow someone that hates behemoth additional options for dealing with it. This change would probably also change behemoth's value.
(just to be clear this isn't a planned change, but is an option I have been considering)
also null synergy - anything nulled can't attack this version of behemoth, lol