This question was originally going to be "Is it possible to earn an ATPL without also ending up with at least one airliner type rating?", but I thought I'd broaden it just a bit (hopefully not too broad).
Requirements for an ATPL in the United States include 1,500 hours total flight time with several minimums in various special conditions and as PIC, "applicable to the aircraft category and class". My question is basically, "what is the definition of 'applicable'?".
For instance, it is theoretically possible, and even useful, to obtain an ATPL in the ASEL class (so the answer to my original question is "yes"; you can earn the FAA's highest certification level without being signed off on anything close to the type of aircraft Southwest or Delta have in their fleet). You could get that license and fly Cessna singles for Ravn Alaska (formerly "Era Alaska" of Discovery Channel fame) or any of the other bush airlines around the country.
However, that represents a lot of flight time before you can even sit the written tests and fly the checkride; 1,500 hours total (250 as PIC), 500 of that XC (100 as PIC), 100 hours night (25 as PIC), and 75 hours real or simulated instrument time (no more than 25 hours in a flight simulator, 50 if you're training under Part 141), all in single-engine fixed-wings. If you then wanted to progress to multis and thus eventually become useful to a major U.S. domestic airline, you would have to start from scratch as a student pilot in the AMEL class.
... Or would you? That's the question; how much of your flight time and/or certification level in one class is "applicable" by FAA regs toward any other class of the same category at the same level of certification? It seems unfair for a pilot with a fairly high ASEL certification to have to start from scratch on flight hours just to fly a plane with floats, or more than one engine, or both. However, they are different classes for a reason; more, or at least different, knowledge and experience is required to taxi a float plane on a lake or to get a multi-engine plane off the flight line. So while there is definitely a lot of overlap between the four "Airplane" classes, it's not 100%.
How about applicability of flight time and certifications to other categories? I'm particularly interested in the "rotorcraft-gyroplane" class and the "powered lift" category, as these were developed as aircraft hybrids which AFAIK have a significant skills overlap with a fixed-wing aircraft. Does flight time in an autogyro count in any capacity towards flight time in a fixed-wing, or vice-versa? How about flying a Harrier? In 50 years when the Osprey makes its way to GA in come capacity, how would existing civilian airplane and/or helicopter pilots get rated to fly it (I hope not by amassing 1,500 hours in an Osprey)? How about something completely different? Even between a fixed-wing and a helicopter there's some overlap (though I'd imagine even the chief pilot of a major airline would be out of his depth in a B204).
