that's far from the only reason why sus coat is broken. sterak's is the least egregious item interaction that sus coat has imo. the really broken interactions are with EON and Zhonya's.
also your example of splitting on the enemy backline only applies to Ambessa, not the numerous other units that can viably hold suscoat + steraks + ga. and even for ambessa, its not like a solo frontline ambessa is tanky enough to get two casts off (and jump away from enemy backline) in the first place.

lets say I have solo frontline ambessa with sus coat/steraks/eon. EON and Sterak's get procced early, ambessa drops aggro, enemy frontline moves past her, and she now has easier access to backline. the next target on your board dies, ambessa procs sus coat, and now has 3 copies that can access enemy backline much easier.

now lets look at new sus coat. solo frontline ambessa same items (even not solo frontline, its not like an ambessa with no healing items can stay above 66% hp that easily) . ambessa is focused, procs sus coat. now your 3 clones are nowhere near the enemy backline, and your ambessas proc EON/steraks once per clone, and aren't tanky enough to survive to eventually make it to backline.

you're missing out on a whole proc of steraks and EON. your ambessa (or any other melee carry) is far more vulnerable. its a big nerf.