That would literally be the most busted thing ever.
A 5v1?
The poor guy would have no chance.
See, with your proposed change, doing well would be the most punishing thing ever;
Say your Renekton is fed.
The guy has a 500 gold bounty, against a Morde who is feeding his ass off.
Morde ults Renekton at the start of a team fight.
Normally, Renekton would punish the fuck out of Morde for that mistake, but this instantly allows the entire enemy team to whale on Renekton with no way for Renekton's team to help him out.
If Morde keeps doing this, he can instantly remove the laner he fed from having any impact in the game, with little to no counterplay, or skill.
The 1v1 mechanic on Morde's ult is to keep it fair; if Morde gets a portion of his fed ass enemy's stats, he shouldn't also be able to force them into fighting a 1v5, where it'd be easy to take out his bounty.