I personally have no opinion if bitcoin is sustainable with transaction fees only, perhaps it is, perhaps it isn't.

But, why do you think that "thousands of smartest people on the planet" have solved this issue, or, even why do you even think that all of them have given this issue much thought at all?

My thoughts are that a vast majority of people simply saw that, hey, mining is hugely profitable right now, why not get involved.  And if it is sustainable after block rewards are gone, who cares what happens in 20 or 30 years. 

What would be your estimate of this percentage: of those thousands people, how many of them are *not* involved with mining?