Your argument is good, but it also serves to advance the argument that using SimpleDB for a new web app is premature optimization.
Yeah, when your app has millions of users and massive amounts of data you might need to adopt sharding or clustering, perhaps giving up the advantages of a huge single-machine database relative to SimpleDB. And, once you have this problem, it might be a good time to contemplate rearchitecting your entire system around something like SimpleDB. But few web apps start off with millions of users and massive amounts of data, and to deliberately avoid using SQL tools just because they won't scale to Facebook-esque levels of traffic is the very definition of "premature optimization".
Yeah, when your app has millions of users and massive amounts of data you might need to adopt sharding or clustering, perhaps giving up the advantages of a huge single-machine database relative to SimpleDB. And, once you have this problem, it might be a good time to contemplate rearchitecting your entire system around something like SimpleDB. But few web apps start off with millions of users and massive amounts of data, and to deliberately avoid using SQL tools just because they won't scale to Facebook-esque levels of traffic is the very definition of "premature optimization".