Garen actually scales far better than he did before.
His only problems right now are that judgement slows him down if he uses it in a creep wave, and that Darius essentially does the same thing but better. But is still stronger than before. Better scaling, offensively and defensively, in exchange for very minor early game damage and durability hits.
If you were running AD runes on the old Garen, then he didn't take any noticeable damage hit at all, only to durability through not getting a free natural 25 armor and MR from Courage - but that flat 20% buff comes out way ahead provided you actually build tanky like you're supposed to.