Sam,When seeking to obtain one's ATP certificate, one doesn't pay for all the training to get there (a few do, but they're doing it wrong).
You need only obtain your commercial certificate. From there you may begin working entry level jobs. Over the course of several years, you may perform freight jobs, flight instruction, and other assignments. As you gain experience and are able to meet the requirements of the ATP certificate, then you may apply for it.
Many pilots hire on at the regional or commuter level without the ATP. They obtain it during their first type rating checkride.
Yes, the pay is dismal; this is aviation. It's often said that the way to make a small fortune in aviation is to start with a large one.
I wouldn't worry about the requirements for major airlines, just yet. Some will require you to have a type rating before being considered for hire, and virtually all will require a four year degree and the ATP certificate. However, one must walk before one runs, and there is a lot of walking to be done before being eligible to apply for a major airline. Plan on 10-15 years to get there.
Chances are that by the time you apply for a major airline, you'll be taking a cut in pay to go to work for them. This is a routine that you'll experience throughout your career, as you move from one employer to another. You'll often take a cut in pay to move to different equipment or operating conditions. It's all part of the game.
As for sponsorship, there are a few airlines that do this. Lufsthansa does it, Cathay does it, as does Air China. Quantas has a cadet program, and so does British Airways. You'll need to refer to the various firms for their entry requirements. In all cases, you'll find them very competitive with very specific entry and attendance requirements.
Most of us end up taking the long route and paying our dues over time. Good luck!