tbavprof,
he present, and the future, of aviation would be a dreadfully boring place if the only options to fly were 70 hours of C150/172, and then a lifetime of A320/B737. But, to each his own.
Once your have achieved >1500 hrs and gained your ATPL, there is nothing to stop you converting onto another type. In fact, post 1500 hours there is no regulatory difference in the career path options.
However, you are totally tied to the aircraft AND operator for that initial 1500 hours.