All fair points. I think you boiled it down to a particularly salient point, though; it's always just going to come down to how strong a writer someone is, and fandom characters are much easier for most people to write simply because it takes away the difficulty that is character-building. It means someone has done all the rough-draft hashing out for you, it means someone has figured out motivations and stuff for you, it means you have a built-in group of people who know about your character already and even have a vested interest in playing with them. Most people in RP get by just fine without doing any super deep introspective thinking about their characters, too, as most of it is laid out by canon or they don't intend to play out anything too focused on character development or whatever, because it's about having fun today instead of building a long-term story –– and that's okay! Most people play just to have fun with friends and not many people see RP as a measure of writing prowess. Fandom characters are great because it unites people on interests and fronts and basically allows people to get invested in other characters a lot faster, whereas even the best-written OCs sort of need time to get to know them because you can't just google them and see if they sound interesting for your character to get to know. And if you're not such a srs business writer with srs plans or whatever, you're just having fun with your vampire OC, and no one really digs your OC because a) you can't hook them and b) they're biased to begin with, then you're in even more trouble.

And I guess that's another concern.

I don't want to say "hell yeah OCs come join us" and then we get like, 1 or 2 and people write them off almost immediately just because they're OCs and not even give them a chance. That's super discouraging.