To go back to the original question...
No, CTC is most definitely not the only way into an airline. It is, however, probably one of the more common routes for newbies at present.
I know plenty of my contemporaries who have gotten into the RHS of a Boeing or Airbus; nearly all of them went instructing/light twin/regional turboprop to get there. A few managed to skip some or all of those steps. Hardly any (of about 20 or so) went the CTC route.
Having said that I did my CPL some years ago now..
I'd never recommend paying for a TR unless you've got a job offer i.e. don't get one on spec.