What you need is “static URLs”, not “static landing pages”. The pages can change as needed, just the URL structure needs to be manageable within your script, allowing for user-friendly/SEO-friendly URLs.
Most popular shopping cart scripts already allow for SEO friendly URLs and when you move to a new platform you just need to make sure that the new script allows for SEO friendly URL structures.
I think it would be less desirable to use static HTML landing pages unless they are created and edited and managed from within the shopping cart CMS, in other words, all the same benefits of dynamically generated pages with caching.
A better approach would be to build in an SEO friendly URL rewriting module like other popular shopping carts scripts currently do now.