The SSMEs were bleeding edge engines when they were built. Moreover, they were strapped to the Shuttle orbiter so every time they were used they were on for an

The SSMEs were bleeding edge engines when they were built. Moreover, they were strapped to the Shuttle orbiter so every time they were used they were on for an entire flight from sea level through the entirety of the atmosphere up through vacuum. Then they spent a week or two in cold soak on orbit. Then they experienced a mach 25 re-entry and the jostling of an unpowered horizontal landing at a few hundred kph. Oh, also, I should mention how difficult it is to optimize an engine for vacuum as well as sea level operation. In a vacuum you want a high expansion ratio nozzle because that maximizes Isp, but it also lowers pressure. That doesn't work if you have to fight against 1 atm of sea level pressure. That alone is actually a big reason why rockets have multiple stages, because the Isp boost of the high-expansion ratio nozzle on the upper stage is an enormous performance win. One way around the problem is to boost the exhaust pressure, enabling a higher expansion ratio nozzle which can still operate at sea level, but that makes the engine even more complex and difficult to manufacture, let alone reuse. So you have the SSME which is a high pressure staged combustion regeneratively cooled LOX/LH2 engine optimized for both sea level and vacuum operation. It's a wonder they were reusable to any degree whatsoever within the first several flights. Even so it took until the mid 1990s for the engine design to fully mature and enter a truly operational phase (vs. prototype phase). Meanwhile, let's look at what SpaceX has on their plate. They have a low pressure (1/3 the psi of the SSME) gas-generator cycle (non-staged combustion) LOX/Kerosene pintle injector engine which uses two different nozzles for sea level and vacuum operation. It's a design with heritage back to an era before color television. It's such a tried-and-true design with so much history that the patents on it expired decades ago. Additionally, in contrast to all the rigmarole an SSME has to go through for each flight the Merlin-1D merely accelerates to a fraction of orbital speed, never leaves the atmosphere, and, in a reuse scenario, is back at the launch site within minutes. No hypersonic atmospheric flight. No re-entry. No bumpy landing. No cold soak. No transition from atmospheric to vacuum operation. It's a vastly simpler engine which is exposed to vastly less strenuous conditions. Again, if you think that the Merlin-1D will face anything approaching the problems of the SSME in reusability then you are either extraordinarily ill-informed or you are peddling propaganda.