These types of things are determined by the market. When Es, PAs, Sharks, and C-Liners were being marketed, the railroads wanted ultra-modern, fancy looking machines to attract riders, like Coke wanted a fancy bottle and Ma Bell wanted fancy stylish phones.

There was an attempt to return to "styled" passenger diesels more than a decade ago with the "California Cab" version of the F59PH (and which the folks at MPI seemed to have looked at when they designed the present MP36 and MP40).

I would love a classic streamlined cab unit diesel order myself... but like Coke cans, cordless phones that look like remotes, and everything else out there were function has determined form, I just don't see it happening. There's never been a railroad that wanted a classic look for revenue (non-cruise-ship) trains. If they were to spend money on styling it would be ultra-modern again, like the California Cab.