I am no expert in history, but aren’t invasive barbarian hordes and multiculturalism the primary arguments used to explain the decline of the Roman Empire?

Indeed, but I would wager that the main proponents of such drastic and ridiculous societal movements also supported or participated in the equally degenerate and culture-eroding faggotry that was rampant in Roman society at various points.

I just believe this is not a particularly complex nor important issue like the JQ, for instance.

You are correct in that it isn't complex, but it definitely is serious. The rate at which young, innocent children are bullied, traumatized, indoctrinated, and turned into eunuchs, freaks, or suicide statistics is not only high by constantly increasing. For Whites to survive we need normal, healthy straight couples. Faggots can't have kids, not naturally at least, and they shoudln't be allowed near kids in any circumstance.

Solving the incel problem, banning LGBT propaganda (including porn) and outlawing public displays of such perversion would see levels of homosexuality decline to irrelevancy.

Yes, but the thing is such acts directly undermine globogomo and therefore we'd need significant regime change to see anything close to such measures, even despite their cursory and common sense nature.