There absolutely is still room for new affiliates, because there are always new products to promote and new audiences to reach. The internet is a vast place with infinite verticals to work in, so instead of just focusing on the biggest affiliate programs in a specific industry I recommend using that one as well as others (and Amazon!) to round out your affiliate offers and thus your revenue.
If you have pop-ups and optins on every page, I would have to think that it would be a lot harder to convince someone to link to it. Especially since the pages where you have your optins (and probably receive the most traffic) are probably the same pages that you have your best content on as well that you'd be hoping to pitch in your outreach campaign.