The 737 has had 14 major recertifications. The aircraft today looks/behaves nothing like the original from the 1960s.
The main motivation for recertifications comes from commercial pressure where if a aircraft is given a new number and not recertified, then the pilots have to be retrained.
Honestly, back when the 737 MAX debacle happened, a lot of consumers claimed that they would stop flying aircrafts if they ran into 737 MAXs. And I don't think it happened in enough numbers - or even enough to make news. Sales went through the roof, everything kept working.
Recertifications are very common. The issue really is is the aircraft is AS different and untested as the old MAXs, and I really can't see that happening again in the next decade or two atleast.