Yeah, i mean the database and the original code... should anyone find an old PHP4 server in a basement, and fire it up... the old one would still run, and be frozen in time... :-)
Personally, I'm more for writing scripts to convert as much of the old content as humanly possible

MySQL6 and PHP6 are in the offing and v5 has been standard now for years. Unless you host it yourself, serving the old content just isn't going to be feasible.