Title? I don't need no stinkin' title.
I'm asking this because I'm trying to fully understand your particular opinion...not because I'm going to do a gotcha...
Can you give me a couple/few concrete examples of why you feel a FR campaign can't be as serious as a Core 4 campaign because it has a lot of races.
Well, some opinions and preferences are just that. Opinions and preferences. I can try to explain, but people also tend to simply justify their opinions after the fact. Also, I feel like I have to dance around because no matter what I say someone will take offense because I simply have a different preference even though it's not a reflection on anyone else.
Anyway, for me? It's just a question of how many races can have a large enough population and regional control to sustain themselves long term. There are many reasons we don't have neanderthal, denosovian's or homo floresiensis running around any more. If a species is adaptable enough to be a playable race, they'll compete with other adaptable races. Competition for the same resources doesn't generally lead to a lot of diversity in the long term. Add in at least a few "monstrous" humanoids and it's even worse.
For that matter, it could be any subset of races as long as it's a fairly low number. It doesn't have to be the "core 4". I think even having 2 intelligent, reasonably technologically advanced species on a planet would be highly unusual.
The other aspect is that I don't see a lot of added value for the extra details I need to carry around in my head. I have a picture of the "core" races, they fit a variety of roles from a narrative perspective. They have history, cultural preferences, general personality traits. An elf "feels" different than a dwarf. With too many races it just becomes a gray morass of races that have little to differentiate them other than mechanical benefits and their costume.
I know, I know. Fantasy and magic can justify all. Doesn't change anything because it's all just like my opinion, man. One that seems to be shared by quite a few people I've played with.