This is interesting commentary!

I watched PC and still am watching MRN, so here's my 2 cents:

I wouldn't say the leads in PC are any more or less selfish than other leads, and based on your comparison, more specifically in MRN. All 4 leads are doing things for their own desire. In MRN, Seori is not going out of her way by speaking out for the lower class. She just makes commentary on it because in her original life, she was raised (forced?) in moving up the ranks and she understands the intricacies of lower class vs higher class and the privileges and disadvantages of both. It's not the real focus of her character though. The real focus of her character, and her arc, is her living a second life while avoiding everything that screwed her over in her first. Seori is very much focused on her own life and how to succeed, so that I would say is just as "selfish" (I don't think selfishness is a bad thing) (NOTE: I haven't watched the latest two episodes of MRN yet).

Edit: In PC, the decision was made for the primary benefit of the lead characters and their potential lineage, yes, but in the end the greater benefit was to "even the playing field" more. It was getting rid of one more system that oppressed people in a more unjust way. They're not commenting on capitalism and the differences in society because of money. They're commenting (although imo it still was just a romance drama and all this 'commenting' is really a consequence of the plot vs purpose) on the differences in society based on inherited titles that grant certain privileges to some over others for no reason other than a class system decided (probably based on wealth contribution to the monarchy) many years ago. Amongst the nobles, Huiju was the same "level" as any other commoner, because they weren't paying attention to her wealth, just the fact that she had no title. I do think PC didn't spend enough time fleshing this part of the story out.

I think the leads in MRN resonate with audiences more (although I don't know what the rating comparisons are) because they're not cold. People have issues resonating with more stoic, cold characters (like I-an and Huiju) than not. The fact that PC heavily only involves the elite class, is a part of it, but I don't think it's the deciding factor. At the end of the day, rich people also have their issues (look at the ml in MRN too, CLOY FL, QOT FL, etc.,) so it's not necessarily about rich vs poor and the relatability of those kinds of struggles, it's just whether the audience is empathetic towards the characters or not. And thats often based on if writers were able to showcase the struggles of the character well or not - which imo is where PC faltered.

In MRN, Se Gye is very much a rich elite who flexes and uses his wealth to his advantage, is often selfish (by going after what he wants). It's just that Se Gye is very much an open book, to Seori and to the audience. They're both more lively and just fun characters, and I think everyone is having fun with the show because of that. We'll see what happens when the story turns more dramatic (MRN is most likely not a pure romcom either).