Is there any reason to justify giving an assassin with an already loaded kit (innate sustain, manaless, triple gapcloser, invisibility) even more kill chaining?
Akali's kit isn't that loaded with stuff.
You can point out any champion doing 10+ different things with their kit. Abilities tend to actually do things, separating champions a fair deal from the purple caster minions.
Saying "overloaded" mostly strikes me as bitterness you lost to an Akali.
Especially considering most abilities do about 2 things - such as damage and applying a -ATK debuff, damage and CC, armor and +AD, damage in an extremely interesting fashion, etc.
Akali's abilities really don't do more than 2 things - Crescent Slash is actually really underwhelming and dull, so it's not like Akali's really standing out with an amaze-balls kit.
As for the actual justification of chaining kills and re-sets:
Marksman can take out a solid 50% of an enemy team's collective HP with solid positioning skills around once every match by accident.
Mages can take out a solid 50% of an enemy team's collective HP with a solid QWER combo because AoE once every match by accident.
Assassins can take out 50% of an enemy team's collective HP with a solid re-set chain once every match by accident.
All 3 of those damage rolls can take out a greater % of an enemy team's collective health when played skillfully (or with a skill miss-match).
If you cap an assassin at 20% of an enemy team's collective HP, then that champion becomes extremely limited in their contributions to the team as a damage-dealer. They can easily fall into Yorick/Urgot territory where they're good in lane and total garbage outside of it, or fall into pre-rework Rengar/Yi terrirtory - where you back-door 24/7 with them, then back door some more.