While I completely agree in principle... in application it's not quite so simple. You're already behind so the opponent will force normally-bad trades, or just deny you farm. "just farm under turret" doesn't apply with many champs nowadays (especially against melee). Darius can just hook you when you try to get the casters.
The biggest issue is that many players don't know what to do once they get behind. They either try to get a few more CS than is safe and then die, sit in lane getting denied most of the cs, or spam the jungler for ganks (which - unless also fed - is usually a bad idea against somebody like a fed/farmed Darius). Sometimes, the best solution is just to concede lane and work with the jg to farm and/or gank. It's not a win, but it's less of a loss. Generally, if you lose to somebody like that in lane, it's partially because they're just stronger than you early, so stay alive and plan to outscale later.