Market forces drove Boeing to rush out a more fuel efficient 737. Even absent regulatory incentives, market forces drive them to want to deliver a 737-dimensioned plane, because a huge selling point for the NG and MAX is that they're still compatible with decades' old infrastructure at rarely-upgraded regional airports.
Bolting large enough engines to deliver the market-desired fuel efficiency on the market-desired airframe dimensions of the MAX necessarily required mounting them so far forward that the entire airframe is fundamentally prone to pulling into a stall, and correcting that is why MCAS exists.
Certification costs are far from the only reason Boeing has never sat down and designed a successor for 737, even though they've done so for numerous other planes -- half the problem with the 737 is that its engineering achilles heel (the incredibly low ground clearance) is simultaneously a key feature to a large portion of the customer base. Correcting it means all of those customers finally upgrading their ground infrastructure, which leads to Airbus suddenly being a viable competitor for those routes.
It was significantly more fuel efficient on the sorts of routes the non-MAX 737 dominated, to the point where airlines and airports were starting to redo their infrastructure to accommodate it because the fuel savings were cheaper in the long run.
Rushing out the MAX let them shore up their advantage in that market — get the fuel savings AND save on the upgrades you won’t need.
Market forces drove Boeing to rush out a more fuel efficient 737. Even absent regulatory incentives, market forces drive them to want to deliver a 737-dimensioned plane, because a huge selling point for the NG and MAX is that they're still compatible with decades' old infrastructure at rarely-upgraded regional airports.
Bolting large enough engines to deliver the market-desired fuel efficiency on the market-desired airframe dimensions of the MAX necessarily required mounting them so far forward that the entire airframe is fundamentally prone to pulling into a stall, and correcting that is why MCAS exists.
Certification costs are far from the only reason Boeing has never sat down and designed a successor for 737, even though they've done so for numerous other planes -- half the problem with the 737 is that its engineering achilles heel (the incredibly low ground clearance) is simultaneously a key feature to a large portion of the customer base. Correcting it means all of those customers finally upgrading their ground infrastructure, which leads to Airbus suddenly being a viable competitor for those routes.