Infinity edge is not the AD deathcap.
It's not. Stop saying that it is, it's insanely annoying. Let's compare their "similar" passives.
Rabadons deathcap grants 35% bonus AP, all the time. That Ap applies to non damage sources like shields, slows, ETC.
Infinity edge grants 50% more crit damage. Just damage, that still needs crit from other items to be effective.
The difference between the two is that Rabadons allows for bonuses to utility, not just raw damage. That might not sound like a huge difference but it really, really is a massive one.
core item for mages
core item for ad assassins
core item for bruisers
core item for on hit/as carries
core item for crit carries
core item for ap assassins