Earlier quoted context omitted.
I'm not sure what your point is. Are you saying that MongoDB is on its way to becoming the default datastore for web apps? Or are you saying that it's not, and the comparison never should have been made in the first place?
I'm saying that there isn't a default datastore for web apps in the same way there isn't a default language for programming. Different data stores do different things and web apps are so varied that certain apps will benefit from NoSQL, some from SQL, and yet others from straight text file storage. I find it tiring that everyone thinks their choice of datastore is the bestforeverythingontheweb datastore.
I think it's pretty well established that MySQL was the default data storage system (I say to include a broad range of systems) for web applications in the open-source world for a good chunk of the last decade.
And there's at least some reason for a default to exist. There are many applications where the author(s) don't have particular data storage/management expertise, and they'll be looking to use the "best practice" or "default" system that everyone else is using. So it sounds entirely reasonably to me that there will, again, exist a default way to store and manage data.
And it also seems natural that various systems will vie for that title, because there are a huge number of potential users there. Others will avoid that title because they want only experienced users to be involved (which I think is misguided, but it seems there are always a few).
So, I agree with you in the strict sense that there's no bestforeverythingontheweb datastore, and it's way too early to assign that title to anyone right now, but striving for broad appeal is certainly a reasonable thing to do.