Sure, but the Merlin 1D is still on the very low end of the spectrum for total thrust, which you seem to ignore. You'll note I only focused on Isp for engines

Sure, but the Merlin 1D is still on the very low end of the spectrum for total thrust, which you seem to ignore. You'll note I only focused on Isp for engines with similar or better thrust than the 1D. And you can't just blindly ignore this. The lower the Isp of the engine the more fuel you need and the worse the thrust-to-weight of the vehicle total ends up being. Having a very light but very low Isp engine will not get you off the ground efficiently. And if your claim to fame is being cost-effective, efficiency is kinda important. Or put another way, the RS-25 at 7,480lb each represents a tiny, tiny fraction of the shuttle's total launch weight of 4,400,000 lbs. Using a 5,000lb engine instead would change nothing. Similarly your claim they only ran at lift off to verify they'd start is wrong. The RS-25 represented 30% of the launch thrust. Even on the fairly light and small Falcon 9, the total vehicle weight is 1,115,200 lbs. The engine weight here is important but far, far from being the single most important attribute. And with a higher Isp engine with enough thrust you could then carry less fuel. Pretty sure you could spend 10,000 lbs on the engines to save 240,000 lbs of fuel (Falcon 9 has ~822,000 lbs of propellant, the RS-25's Isp is +30% the 1D at sea level)