So, after a long wait, I finally managed to play the full version of the game.
The wait was well worth it, I had a blast, you really did an excellent job on the game. It also runs smooth, never crashed while playing, no real bugs experienced, and I was really impressed by starting a new character and seeing how much better the first two acts looked. Congratulations 
This also means that I finally got to see the location where the arch of the original screenshot was taken from (also amazing to see the changes in all these years).
At the moment, from what I can see, the arch can still be destroyed… that part is still in the game… the only thing that is missing is the actual effect after the fall…
Now I don’t want to go back to the part about the other destructible objects, the kind of damage calculations required for the debris etc, you already explained well enough how complicated it was to find the right way to do it.
However, in the specific case of the arch, I wonder why it can’t work like for example the bombs you can find on the ground in the plains of strife (loved the location and the bombardments by the way)? It just takes a % of the monster/your overall health if you are hit, scaling with the difficulty level.
Maybe you could consider adding a stun/slow or similar debuff for a few seconds, that’s it. (sort of what it also works in diablo 3 I guess?)
I mean that if a formula to make every destructible object’s shattering can’t work, you can still create specific kinds of these, like the above mentioned bombs, the spikes in another level (can’t remember which) or the archs in this case. Best part compared to diablo 3 though, you can’t actually “highlight the trap” but have to aim at it/ avoid it, and it can also damage you, not only the enemies.
I hope it wouldn’t take too much effort to add some damage to something that is already in the game?
I am insisting so much in the matter because personally speaking, I think that having different ways of killing and things to be wary of when fighting can really spice things up and add more tactical awareness to the fights, which would be further reinforced in this game thanks to your beautifully hand drawn maps.