Who is to say that calculations cannot be a thing with equipment?
Pokemon was able to to do just that. The only difference in this case is that we do not have enough info on equipment as of now, which leads people to believe that it could change a character drastically without consequence.
In reality, all that needs to be done is research on the subject. If we can find a ratio of how the equipment influence a player, Then figuring out when to kill them will be a breeze.
I agree with you that figuring out how each stat effect a character is possible but should it be expect of every single player to either memorize the conversion or carry a notepad to play smash at a decent level? Something like that should be voluntary not expected. The other thing you have to consider is the mental strain it would cause the player.
Here's a example using Brawl numbers if this system was included with a few reasonable problems
To keep things simple lets say 1 defense = 5% more to the normal kill percent and 1 attack = 3% more kill power (if attack is equal to defense that would make defense or attack useless so it has to be either higher or lower)
I will be playing as Sheik vs a Meta Knight on halberd, with me having 4 Attack buff on my usmash and meta knight having 4 defense. Using my prior knowledge of the normal settings I know that Meta Knight can live from a fresh usmash until 93% with good di on a neutral stage like FD, SV, and BF but Halberd has a lower ceiling so i shave off 10%, easy enough right
93-10=83%
Okay now lets add the buff/debuffs, first attack then defense
93-10=83-12=71+20=91
Cool after three quick calculations we know what percentage meta knight can die at, oh ****... I got greedy and use dacus a bit too close to my killing percents or I got the double hit and now it's going to be next to impossible to make sure it will be fresh when I need it to kill... Okay..no problem, typically with the decay system you can just add 15% onto Sheik Usmash killing %
So 93-10=83-12=71+30? ...wait, did he have 4 or 6 defense? ****, most meta knights use 6 but I think this guy has more speed then normal so I guess he has to have 4 or 5? No, that was last round, last round he had more speed... I could of sworn I saw him change it. ****, now that I think about it didn't he switch around a lot of his numbers before finally setting on what he wanted. Why didn't I pay more attention? It's 6.
93-10=83-12=71+30=101+15=116
116%! **** if it's that high I might as well wait a bit more so I can use a safer means to kill like nair, vanish or even side hit usmash instead of fishing for grabs and dacus sweet spots and decay ftilt into usmash, that's not even possible anymore.
MEANWHILE as I was thinking about all this complex crap I've falling into a rolling pattern of always rolling away from the ledge when I techs, the meta knight with his herp derp speed defense built picks up on this easy to see fact and greatly punishes me for my predictable rolling causing me to lose the match because I was thinking too deeply about other things.