First off, I agree that a major challenge in adapting Battletech to a platform like MWO is either pruning away or converting the way damage was dealt and that armor values, damage transference, and most everything damage related were balanced off of that randomness. Finding an acceptable workaround or alteration is going to be
extremely difficult and any change to such a system is going to take
a lot of time. Flaming the devs might make you feel better, but it's not going to make changes happen.
Anyway.
I had always assumed in TT that excessive heat (ignoring the part where the pilot risks actually cooking themselves) disrupted a mech's systems,
ie your HUD goes bonkers. Your computer has trouble finding locks, your crosshair(s) move erratically, that kind of thing. I'm also still amazed that auto cannons have either zero or negligible spread, especially past their optimal range. You have to compensate for a tiny bit of fall-off, sure, but there's no variation otherwise. Additionally, considering increased weapon spread was one of the
drawbacks of UACs, not needing to deal with a slug -- that is quite literally powered by an explosion -- going exactly where you need it strikes me as a curious omission.
I know a lot of people hate the idea of your crosshair != where your shot goes, but at the same time that's usually a core part of any other game that is
in some way a shooter. Both bullet spread and weapon sway (or at least increased spread) while moving are two things that are almost always present.
There's also a multitude of ways to implement the aforementioned 20+ damage having a chance to knock a mech down from TT. It doesn't need to mean you
actually fall over, it could be a much more severe rocking of the mech's torso (think of what an Atlas does when you hit it with an AC/20), or, I don't know, they could actually use the fact that you can look around your cockpit and force the pilot's PoV to not be glued to the center piece of glass in the cockpit for a couple seconds.
There are cues that can be taken from flight simulators as well that could introduce difficulty under certain circumstances: think red-out and black-out. Things that aren't adding RNG onto where and how well you hit something, or forcing changes on a weapon's attributes, but that instead make it harder for the player, as a human, to accomplish exactly what they want without making it impossible.
Really all I'm trying to say is that, yes things like heat could still use some changes, but there are a huge number of ways to alter gameplay or penalize certain actions without introducing arbitrariness to the game's core mechanics.