^I don't think it was that he stayed
, as Daryl is so much younger. I think what he said was something like "He did that you, too?" and sounded rather surprised.
The difference between Merle and Daryl is that Daryl doesn't seem to enjoy hurting people or inflicting pain. He WANTS to help, and however much he hates to admit it, he does want to make a connection with someone else. He didn't have to be kind to Carol when Sophia was missing but he felt compelled to do.
Merle seems to revel in being cruel. He'll sacrifice almost anyone without the slightest hesitation. Let's face it; he's a mean bastard. Daryl is seriously badass, but he's not mean. At his core, he's actually a good person. Daryl is brave and Merle, while one badass MoFo, is actually kind of a coward. He picks on the little guy, is too scared to go after Michonne in the forest, and won't stand up to the Governor. He's an opportunist and a bully and even mocks Daryl, the only person in the world who gives a crap if he lives or dies.
You most certainly cannot say that Merle could every replace Daryl. They are not the slightest bit the same. It's like saying Carol is just like Lori, because they are both female.