Trample is kind of a pain. Its a set of rules where the ‘obvious’ play doesn’t interact well with the physical representation of the game. Remembering to trample over isn’t a skill we’re interested in testing, but we don’t want to see that damage resurface out of nowhere after strategic decisions may have been made involving the current life totals.
People ask why we don’t have a defined shortcut, especially since we used to, but that misses the point of why we have predefined shortcuts generally. The predefined ones are there to prevent the player controlling the current flow from using ambiguous communication that they can then later exploit. The old trample shortcut was “you do the minimum damage to everything and keep going”. Why was that? Consider:
I attack with a 5/5 trampler
I block your 5/5 trampler with this 2/2.
Damage on the stack?
Sure. I’ll Giant Growth this guy.
OK. I’m dealing all 5 damage to the creature.
See the sneakiness here? The defender assumed the damage was happening 2-3 and acted accordingly. The attacker got to benefit from not being clear and had incentive to try it again some other time (and they certainly did). The shortcut put an end to that.
Then M10 came along and, along with all the other rule cleanups, made the shortcut pointless. Run the above scenario again without damage on the stack and you’ll quickly see that without a gap between damage assignment and damage resolution, there’s nothing left to exploit. That’s certainly a win for the new rules. So, eventually (since nothing moves that fast in policyland), we got rid of it.
So, what does that mean for when we get called to a table because of a life total dispute arising from someone remembering trample? Honestly, any kind of rule would be pretty terrible. You can’t default to “player sops up excess”, because then you run into situations someone realizes that they forgot trample a turn later and the other guy has done something that makes them now dead. You don’t want to codify maximizing damage, because someone will inevitably claim that you did all 5 damage to the first of the 5 1/1s blocking, because you failed to say anything else. So, we do the only thing possible: default to what actually changed in the game. How many creatures went to the graveyard? Did they say anything like “and you take two” within a reasonable time frame? Did life totals get written and acknowledged – a requirement that was partly inspired by trample? This makes everything work well in the context of the game – the net effect is almost always “creatures die, opponent takes damage if it gets pointed out” – even though a player will occasionally forget some damage and be upset a little later that they didn’t get it. Ultimately, it’s the responsibility of the person controlling the trampler to be clear, and that’s where we want the incentives.