no doubt zouma is good, but i think you're overestimating his talent due to the side he's played on. if he were playing at a mid/bottom prem side i don't think you'd be thinking he's the world beater he is. matic helps a lot, as does having hazard and costa putting in goals.
the rest of those are just what you said, promising, but they couldn't replace terry tomorrow up to mou's standard if he fell off form. i get your point, but i don't think they're in just to fill a quota, otherwise they'd grab up cheap players like city did with delph and roberts to just fill out the quota. they're looking at stones as terry's replacement and zouma will replace cahill (turns 30 this year, he won't last forever either).