Realize his theory was replacing ether theory. As people learned more, ether theory required increasingly arbitrary “patches” to work. If GR was not simpler then Ether theory, it wasn’t a good candidate to replace it, as lorentzian transformations in ether theory still worked mathematically.
I can see why Einstein would assume 1), 2) and 4), but what was his motivation for assuming 3)? Just some intuition about simplicity?
Realize his theory was replacing ether theory. As people learned more, ether theory required increasingly arbitrary “patches” to work. If GR was not simpler then Ether theory, it wasn’t a good candidate to replace it, as lorentzian transformations in ether theory still worked mathematically.