They are already doing that, look at what Primal is doing. Be on X and advertise their client. Primal also sells monthly subscriptions to pay for costs. Eventually as they get more users they will add ads on their client. That new @Nostrich client which is basically a copy of X, is doing the same. Although they advertise as 100% free right now, they are playing the corporate fiat game. Free for now, when they get enough users they will add ads and they already said their goal is to be a public company. The issue is they are competing against X and Facebook. You will need big money in order to compete against them and the nostr user base isn't big enough to generate any decent money and most don't want to pay a monthly subscription for social media. Plus we have amethyst. It's free.