While I don't disagree, the migration for existing installations to something of this nature would be a disaster waiting to happen.
Considering the scope of changes which would be required, as in almost every query in the core that deals with the blog itself, there wouldn't be a feasible way to do so.
If you want and MU style site in a single table fashion, there is an alternative. Granted that last I looked a few months ago it was only synced with the WP 2.3.x, but hey, who cares about all the holes and new features since 2.3 right?
My point being with the reference, intentionally left unnamed, is that upkeep on something like that isn't easy and takes a lot of time.
MU is what it is. You either work with the tools given, and do your thing, make your own, or skip it all together. Such is life.
And no, he was estimating 50k blogs, which from experience is a pretty lofty goal and real world values may be less. They may not be, and thinking ahead is good, but in the end, multiple databases spread out isn't much different than sharding. The end result becomes quite similar. The main difference is that the pieces are smaller, and easier to move around.
Take the low use blogs, group them all into a single database since they're rarely accessed. Spread the resulting savings around to the blogs that are used.
Really, this debate is one that extends beyond MU as well.
There's more than one way to do things. MU does it one way, which may not be everyone's preference, but it's not like it's a broken, world ending crisis either, and something completely unmanageable that has no hope for more than a handful of small sites.
I'll again point back to wordpress.com. Millions of blogs, and they use MU. Granted they've customized it, but who hasn't? Point being, it works. It might not be someones preferred way, but that doesn't mean it's wrong and the world has to come to an end.
To add to that, it's nothing a good DBA can't handle. Sure, it may go against their training but it is something that is quite capable and manageable. If they're any good, they'll be able to work with it.