Especially because with how the game is structured, getting your Magikarp murdered as soon as you get to the idling part is 'optimal'

It honestly doesn't really end up that way because you get so much Trainer EXP for letting your Magikarp safely retire, and that's the main progression gate to get better Magikarps are clear the League.

I might be misremembering but I thought you still got Trainer XP if your Magikarp died, you just didnt get the (trivial) motivation bonus for levelling your next Magikarp. So if your current Magikarp's potential wouldn't clear a league but your next one would, it was beneficial to murder the current one ASAP than waste time levelling it to max so you get the league rewards faster which is more impactful than a temporary 10% XP hit. I do remember intentionally murdering my Magikarp for such reasons but maybe I am actually wrong to do so?