100% crit without armor pen will net more damage overall than a lower crit chance with armor pen. However, if you're over 50% crit chance, you might as well split the difference - especially since it takes very specific items to hit 100% crit.
However, not having lifesteal makes you extremely vulnerable, and if the team starts rockin thorns, you'll end up killing yourself. You won't be mitigating much of the magic damage, and the 30% magic damage return is based on your full damage before mitigation. In other words, if you hit someone for 1000, you'll take 300 damage, but they may take 500 or less from the hit depending on their mitigation.
Before they changed the sword with grieving wounds, it was possible to have 10% lifesteal and still hit that 100% crit chance.
These days, you're better off fitting a BT - the last whisper is situational since the meta appears to be moving to health stacking with resists lacking their defensive utility this season.