There have always been solo pilot planes. Including, for some time, Cessna Citations.

How do the cockpits of solo pilot Citations compare with the Citations not allowed for solo pilots?

And how do cockpits of two pilot Citations compare with cockpits of two pilot commercial airliners?

Embraer now produces solo pilot Phenom 300. And it is somehow not limited to 5700 kg: it has MTOW of 7951 kg.

And Embraer feels qualified to produce solo pilot airliners:
http://www.spiegel.de/international/world/0,1518,716225,00.html

Could Embraer next launch 24 ton MTOW Legacy 700 and ERJ145SP, with Phenom 300 cockpit?

Michael O´Leary, of Ryanair, wants to dump copilots and let stewardesses land aircraft in emergency. Do stewardesses now land Cessna Citations and Phenom 300s in emergency? If not, who do?

Ryanair wanted to buy 200 737-s - Boeing refused. Ryanair wanted to buy 300 320-s - Airbus does not want to sell either.

If Embraer were to build a solo pilot ERJ145, would O´Leary buy and fly 400?

There aren't too many Cessna Citation stewardesses, or Phenom 300 or even 500 stewardesses out there.  Mostly because Cessna and Embraer do not make planes that are able to fly intercontinentally (when business passengers are more likely to need the services of a stewardess).  So my guess is that because single pilot airplanes don't fly that long or far, the chances of needing a second pilot are far less than that of say a 747 or A340. 

Another reason planes have a single pilot rather than two pilots is company policy.  Take for instance the difference between the two company's here in Wichita who fly air evacuation in King Airs.  One has 2 pilots, the other has 1.  The only difference is company policy.

The final reason an aircraft has one or two pilots is aircraft complexity.  A 757, 737, and even a Lear 35 require 2 pilots simply because of all the procedures which must be done during the course of one flight.