Re: Climate change?
Because Raptor can't throttle down far enough. A single raptor at minimum throttle (40%) still puts out nearly the same power (750kN or so) as a Merlin at full thrust (850kN roughly)
The landing burn is done on a single Merlin at nearly the bottom of the throttle range (500kN or so) and even then it's cutting it close in terms of lighting the engine and performing the divert maneuver to actually reach the barge or pad (since the rocket aims just to the side during the free-flight phase of the return so as not to hit it at "oops the rocket didn't work" velocities). That "hover slam" maneuver is designed to reach as close to zero fuel, at zero feet altitude and zero velocity as possible. For a rocket like F9 in that weigh/performance class the weight ratios can't really change, so with a raptor engine with much more thrust the time between lighting the engine and the rocket coming to a dead stop (or rising up again) are too close together to light the engine, divert course to end up on the barge/pad and come to a stop at 0 altitude without some seriously risky aggressive maneuvering angles. Plus lighting an engine at that higher thrust level also puts more forces and stresses on the air frame, which the structure might not like very much.
The whole point of F9 is that the engines are no longer the super expensive bit either, since they don't get thrown away after every launch. Sure you could maybe stop the assembly line of Merlin engines, but that line is basically on "tick-over" as it doesn't need to supply new engines for every launch. The expensive bit for engine production has been done already anyway. The production line exists, the engineering is done. Other than a continued cost of personnel and upkeep keeping it going is not super expensive anymore compared to the umpteen million spent on building the line in the first place.
Redesigning F9 to take Raptors is probably not going to save more in Merlin production line costs than it costs in engineering time investment alone to redesign F9 from the ground up to take Raptor engines. That is the paradox of engineering. Sometimes it's cheaper NOT to do something, even though every fiber in your being as an engineer SCREAMS that something can be done cheaper, better, faster and more reliable. If you're only going to be building maximum 30 of something in the next 10 years, and you might save 50.000 per unit (so roughly 1.5 million in total) but it costs you 2 million in engineering and tooling costs, does it make sense? Only if you expect to keep building the thing for the next 30 or more years.