@Meybi
Well, that significantly depends on the narrative style. From what I've seen, the significant majority of romance manga (with female MCs) actually favor a single love interest. Stories with two love interests are by no means uncommon, but even those typically highlight a "favored" interest, and only rarely pull a twist at the end.
Though I can't argue that multi-love-interest narratives typically overplay their plot gimmick, nor can I argue against the existence of certain manga which base entirely around overemphasizing that concept. Still, in those cases it's usually not that the plot itself goes "nowhere", but that the plot development is trite and bothersome and needlessly stretched out. That happens plenty with "will-they/won't-they" or similar single love interest narratives as well. In fact, it's reasonably common across any theme, especially for manga which become popular enough to get by with "filler" releases.
It ends up being more of an issue inherent to stories which intentionally drag their plot, stories which put romance secondhand [eg, to action], or soap opera-flavored romance manga, than it is an issue of stories emphasizing a particular number of love interests.
Of course, none of those have anything on those rare Sports manga with romance elements.
100-something chapters per reference to their relationship is honestly down to the point of it just being lip-service.
In any case, bit premature to bring that point up for this narrative, given that the current cover only shows her, a single presumed love interest, and her cousin. I mean, unless you're assuming a certain something, but I think the fact that this first chapter has the cousin claiming to be eloping [with a third party] kind of kills that assumption before it can legitimately form.
Though, if such a thing was a trend for her family, it could have some interesting implications as to why she could use magic, I guess. >.>
"Thanks to my special lineage, I have scales, a tail, and can breathe fire!"
"..are you sure you're a mage and not, like, a dragon?"