As a PHP dev with ~100 WP installs under my belt and plenty of customization, I think I'm qualified to say that WordPress isn't written to be scalable. It's actually kind of crap. Many of the things it does to make writing plugins easier for newbies are Very Bad Things in PHP. WordPress is a memory hog, to the point that foreaching over query data in the wrong way can cause you to hit the memory limit, even if just unwinding your foreach into a copy/paste wouldn't. The memory leaks are somewhat nonsensical, and they make scripting with the WordPress API a minefield.
I'm not an expert on WordPress internals, but the scene is definitely ripe for a replacement simple due to the quality of the API. WordPress has been good enough for most people for a long time, but it has many weak points.
IMHO the problem is that's old technology, who used to write free software N years ago is now busy doing startups ;) So the "next generation" of free software web stuff is missing in part.
I'm not an expert on WordPress internals, but the scene is definitely ripe for a replacement simple due to the quality of the API. WordPress has been good enough for most people for a long time, but it has many weak points.