Heh, point taken. In answer to your question, there are other factors in why people don't just take the TR and leave: Training bonds, the need to build competitive flight time, the lack of immediate prospects, comfort level in a current job, etc. Privately funded TR courses are bloody expensive, and your average 1st year regional FO simply doesn't have that kind of cash laying around. Plus, there are the usual right-to-work issues that I alluded to when looking for a job outside your home country. My long-term plans all take me outside of the US, and I'd be willing to pay for a PIC type if that was the only way to get on with another carrier. Or I could just wait around for my upgrade and change jobs at my leisure. I was being a bit facetious, but the purpose of the SIC type in America actually is to make the recipient of the TR less marketable for competitive job offers. Most major airlines will PIC type both pilots, but very few regionals do. It's enough to make one quite bitter...
To answer your question about the hypothetical 727s: Legally speaking, to fly right seat in such an operation domestically in the US the FAA only requires a Commercial/Multi Engine certificate. Only when you go international do you require a SIC type. As galaxy flyer mentioned, the actual legal requirements for this type are very basic. That said, most operations will not simply award one based only on the applicant's ability to fog a pane of glass. The one at my company is not exactly a picnic - we are expected to perform to PIC standards, and the checkride is a jeopardy situation.