Curious, where do you think datomic and datalog fall short (assuming one is using clojure and datomic's performance is tolerable)? By "perfect attribute-oriented model" I assume you are referring to the database's information model and support for that model, not to domain modeling with triples. Triples and relations are equally flexible but which models require more effort to maintain and transform along side with your application?

I think Hickey's point is that the structurally rigid nature of relations and stuff like arbitrary join tables needed to created many-to-many relations is that they get hard-coded throughout your application making your applications very hard to change over time, forcing you to put extreme effort to provide a set of logical views to isolate the application from the physical structural decisions. He says 'the more structural components(tables/intersection tables and having to name them(places)) you have in your model the more rigidity you get in your applications, but as you mentioned, triples haven't prove themselves.

I'm not experienced enough to understand all the tradeoffs here and can't tell if Hickey is wearing a salesman hat ;)

Of course, databases like postgres would be my first choice for most projects (even if using clojure which I like but is still a hard sell for webapps), as you said, they have too much going for them (flexibility of relational algebra, SQL, they are well understood, widespread, great ahd well supported implementations, etc..)