What potential downsides do I need to consider when creating a membership site?

I have an email list of about 15,000 for my blog that teaches people how to use frequent flyer miles to fly free. I’ve tried many ways to monetize, including courses, webinars, products, advertising and affiliate income, and while people buy things, it’s not the level of success I want after 4 years of trial and error. My ad and affiliate income (where I generated most of my income; reached $10K/month this year) has currently dried up to next to nothing due to the economy.

I provide a lot of valuable information to my readers for free. I’m thinking about putting the top third of that valuable content inside a membership site, as well as my 3 courses, video trainings, etc and charge a monthly fee. but I’m afraid I’m overlooking a downside. People can still be on my list and still get plenty of good content if they don’t sign up, so it shouldn’t result in a big drop in subscribers. I’m already creating the content, so it’s not any new work. But if not enough people sign up for this, and I still need to generate more income, does it prevent me from selling my courses separately again since they are now part of the membership site? What else might I be missing as a downside? Thanks!