In the late 1980's through mid 1990's it was common for sponsored pilots to join BA (and others in Europe - Lufthansa etc) with under 300TT straight onto B737, B757 and A320 aircraft.
It still is today, although nowadays many sponsor themselves to an fATPL/IR and join Easyjet or Ryanair, sometimes paying for their own TR etc.
Going back to the 1960's, BA (or BOAC as it was then) had B707 skippers in their late 20's (as did BEA - shorthaul in BA with skippers on the Trident [think B727]).
The system has worked for BA and LH etc, not many burning holes in the ground.
I joined BA with about 300TT, but not as a sponsored cadet, on the B737-200 striaght after my IR which was on a GA7 Cougar.
I gained my B737 with 3200 TT and flew with guys who had been laid off for 3 years and kept their hand in with the occasional sim detail. They were absolutely fine.
It is about the quality if the training rather than the hours.