Imho, the way solve this slippery slope issue is to have certain units which under specific circumstances can annihilate several times their cost in enemy units.
What's a nuke? A Krogoth? An OTA MRPC or HLT? A Merl, for that matter? Yup, they're all things, in OTA at least, that could do what you're describing.
There is no real solution to slippery slope. At some point or another, the game needs to tip towards inevitable victory, with only extreme situations (
commander death, anybody?) or completely erroneous play on the part of the player that's winning changing that outcome. Otherwise, it ain't much of a game. As much as people hate losing, that's just how it is.
The real issues are:
1. "Why do we get there?"
2. "Is how we get there fun?"
3. "How fast do we get there?"
4. "Can any chain of events in combat reverse the trend?"
Question 4 gets into what you're talking about, which is per-unit balance- that "rock, paper, scissors" stuff of units and their counters which is at the heart of most RTS game designs.
However, most of this discussion is concerned with questions 1-3, although I'm still waiting for somebody to seriously discuss 2, instead of avoiding the issue entirely. After all, it doesn't matter if 1, 3 and 4 are all firing on all cylinders, if 2 isn't present.
And saying, "fun's relative" is just a cop-out. Players obviously find some things more fun than others- we can see sales numbers to prove it. The real question is, "why are some mechanics considered fun?" imo. Or, "why did StarCraft out-sell OTA by huge margins?", if ya wanna get to the point.
My feeling, based on what's been happening with P.U.R.E., is that StarCraft out-sold OTA by huge margins because
less stuff was completely automated.