They simply stop receiving regular, automated emails. They remain on your list, but they will only receive emails if you send out “broadcast” style emails (that go to everyone on your list at the same time).
This isn’t such a big deal since you will typically make most of your money from subscribers during the first few months. Over time people become a bit less responsive, so adding more emails to the end of your sequence might not give you particularly good returns.
Having said that, Mark has subscribers who have been on his lists for years and keep buying!
If you do decide to add more content to the end of your sequence, you could see if there is a related niche in AffiloJetpack, and weave some of the emails from that niche into your sequence. (Remember to set up your affiliate redirects first.)
You could also hire someone to create additional content for you.