More Words, Deeper Hole

That contains a lot of dangerous assumptions. First it is rather like assuming that the vast majority of humans in ancient times preferred farming to hunting & gathering. Farming does support larger populations, but it isn't necessarily the better lifestyle from the individual point of view.

Likewise you are assuming that lots of inland seaways would have the same atractiveness or productivity of the natural coast. Instead of (possibly) making for climates rather like that of the area of the 'lake effect' that results in the cold and deeply snowy winters and hot and muggy summers of upstate New York.

And finally you might not know that many of the fastest growing counties in the US are in the dry interior states such as Colorado, Nevada, and Utah. The 5 fastest growing states are Nevada, Arizona, Utah, Idaho, and Georgia. Only one of those is a coastal state. I think a lot more people would like to live in the dry interior than currently live here due to history and technological/ecological limits.