As you mention in your post "As far as the entertainment consumer segment (particularly those who are on desktops) goes, a tablet makes more sense for them than a laptop. " That is a huge segment of the population. So, if that segment goes missing or even partially goes for tablets, the total number of PC's shipped will go down in a big way. Atleast, PC sales can't grow from here.
Also, as the tablets become more and more powerful, one should be able to run more memory intensive applications.
I have seen a couple of people around me whose overall PC usage has gone down drastically after owning a Galaxy S2 or iPhone. Not is disagreement with your points but I must say that if we are not yet in the Post P.C era, we are fast moving towards one.
Desktop segment is bound to decline over time, yes. But the laptop market is not going down anytime soon. Infact they will see a surge for the next few years. So the overall PC segment will still hold forte. Having said that, one would love to see more powerful tablets and smartphones. My point being: stating that its a post PC world is far from the truth. And yes, users perhaps in all segments will have a distributed time spent between their PCs and these new form factors.
The point is that even the overall sales of laptops has to go down as people have more options.