> Einstein saw the whole forest, while Heaviside and Thomson were looking through a magnifying glass at the bark on a single tree.

That's an incorrect characterization.

Heaviside is pre-Michelson-Morley. Einstein is post.

The fact that "Ether doesn't exist. Electromagnetic waves simply transmit without a medium." is a VERY VERY VERY big change in the fundamentals of physics.

That opened the door for the hypothesis that "all physical laws look the same in all inertial frames" because without "Ether" you cannot anoint a preferred inertial frame. The Michelson-Morley experiment was the first test of that hypothesis.

And, to be fair, the kind of fields interpretation that Einstein championed has weaknesses:

1) It's an absolute bear to compute--the Heaviside-Hertz formulations lend themselves to engineering much better pre-computers.

2) It had severe experimental holes--at the time.

For example, the Einstein fields interpretation makes the prediction that an an excited electron in an atom will never decay. That, of course, was nonsense--excited atoms always decay to ground state. Except--now that we can isolate single atoms we absolutely see that single atoms don't decay--only the ensemble interactions cause them to decay.