If'n you don't solo in 15 hours, either you have a learning problem, or your instructor has a teaching problem, or your school has an ethical problem.
In the days when all true training airplanes had the third wheel at the back end, 8 to 10 hours was the norm to solo; 12 was for below average/slow learners. At 15 hours if you had not gone solo, the honest instructors - who were mostly ex-air force - would have discreetly suggested you seek an alternative career.
Maybe taildraggers were easier to fly back then? I think not, So, nowadays there's a bit more to learn about radio etc, but mostly the fad for delaying first solo seems to be about instructors who need to take the extra money for dual. Again, in the good old days there was only one hourly rate for learning to fly. Dual or solo rates did not come into the equation. Presumably the schools had averaged it all out so that their instructors got a weekly wage rather than an hourly rate.