Hi Sarah,
I download the HTML for the email (by clicking the View as a webpage) and view the source code, copy and put it onto a blank template landing page in Marketo. Then I link my website newsletter archives to each landing page instead. http://www.leandatainc.com/news.html
I do this to track clicks but you can take the HTML and put it onto a blank website template too, and host it yourself if you are concerned. I do not know of the landing page not being accessible or having a deadline date. (Ony if I cancel Marketo). I believe I can still archive the program, and the landing page will still be accessible.