[Silhouette]

I think it's taken as a given that your app won't be sitting still for five years.

Many, many web apps will do exactly that, just as many businesses are still getting work done with desktop software that is several years old. But in the world of web, browsers now break functionality all the time and vulnerabilities are found with alarming frequency, so the ability to update front-end code to at least maintain compatibility and security is often important, even if it's otherwise working fine and needing little new development.