Lyxen
Great Old One
As I know about the number of encounters we have in a session I'd prefer something less abstract and not have to just wing it as @EzekielRaiden put it. I'd like a little more precision to be confident that the encounters I create are close to what I expect them to be and adequately challenge the players to the level I want. If I want an easy or deadly encounter (or something in between) the guidelines/rules of the game should be able to accommodate that within reason. After all combat is a pretty integral part of the game and should be more than a crap shoot.
Combat works fine, it's quick, streamlined and exciting. What it's not is challenging, and that is the whole problem for a number of people, including you, I suppose.
To create a challenging encounter that does not have the risk to turn into a TPK, you need precise computations of power, something that you cannot get out of 5e because the monsters are not calibrated and because the situations are not calibrated. 3e sort of had it, 4e got it down to near perfection, but only because it was a "closed" system in which the possibilities were restricted and computations could be made. 5e is way more open, with some advantages and some drawbacks, in particular in terms of precision.
And then, once more, if it was doable, don't you think that someone would have done it ?What I think is that if they could do it for 3E and 4E as you said that it should be possible to get it a lot closer in 5E than it currently is. Im not looking for perfection, just something close and more reliable than guessing most of the time.