We don't need a RDBMS; we get along quite well with key/value + keyscans, making it a lot easier to switch to a database with roughly similar features later (CouchBase, Cassandra, HBase, possibly others)
OK, slightly ignorant question here, but SQL is kind of a standard for RDBMS's. As far as I am aware no similar starts exists in the NoSQL world, so I would have assumed that it would make switching harder. If you are just using it for basic stuff, then "SELECT * FROM table" is pretty standard across RDBMS's.
Reading this:
(And in half a year's time, the database will have outgrown whatever hardware we can reasonably throw at it, so then we'll have to start over for real...)
You need a rdbms. Field names won't be repeated for each query and you will push later the time to 'start over'.