http://arstechnica.com/information-technology/2014/06/how-to-design-a-web-application-with-a-40-year-lifetime/

GrandmasterB:

think it's a bit unreasonable to expect a Web application circa 2013 to be still up and runnable in 2053... So data is your primary focus. As long as your data is still there, people will be able to adapt to whatever new technologies come about. Make sure your data schemes are well thought out and well suitable for expansion. Take your time spec'ing them out...

Tangurena:

...you should be concerned about logging everything and having audit tables that track changes/deletes in data. When upgrading versions, plan on testing new versions in parallel for enough time to ensure that you've got your data migrated. Preserve installation media and license keys in the event that a rollback needs doing. Test backups. If you are going to serialize objects to store in the database, do so as XML instead of the serialization supplied by your development framework... you need to document everything in something like a wiki... For your codebase, make sure you have comments in your code...

ChrisSD:

How do you design a web app to last 40+ years? You don't. You can't...

cpragman:

If you use something standard enough for the data engine, you should be able to keep it running forever...

wolfonenet:

Document the reasons behind EVERYTHING... Keep the code heavily commented, and the comments up to date, and make sure you [#ff0011]concentrate on the WHY not the HOW[/#ff0011]...