Nic while i like your logics in general but here I see sheer pessimistic approach to the scenario which will happen after 12 years. They (Qatar) have achieved whatever they planned sofar and I think it’s comparatively more sustainable than other equals. In the same logic, if they plan something big in future they’ll achieve that too.
Your assumption, on managing such gigantic works (is that really gigantic???? Mere 50 Bn in 12 years. hardly 4 Bn per year????) and population mix are, it seems, based on fantasy, you even missed population growth in future (exponential not linear).
As far post event utilization of infrastructure, that will serve their country and obviously be in line with their future needs.
I find they are using their wealth in right cause and betterment of their citizen (directly) and other people (indirectly) instead saving it and buying leisure islands !!!

I find they are using their wealth in right cause and betterment of their citizen (directly) and other people (indirectly) instead saving it and buying leaisure islands !!!
-----------------