Isn't "it's comparatively slow" what people usually mean with "doesn't scale"? You can scale anything with enough hardware, but as you mentioned at some point is just becomes very expensive.
Twitters architecture at the time was a textbook example of how not to build a large scale many-to-many social network. Maybe switching would've been worth it for them anyway, but the big thing they needed was fixing architectural choices they never should've made to start with.