Uh... no?
You're confusing reality with human understanding. From the moment Einstein wrote down his theory, it was either right or wrong as a fact of nature; and apparently he was right, or very close to it. The other competing theories were not.
The fact that it took us decades to confirm that he was right is irrelevant; Einstein did instantly and immediately (or, well, over the course of seven years, but on the scales we're talking about that might as well be 'instant', that's the timescale of an average PhD degree) come up with the right answer.
And actually yeah, it sort of is him being "some galaxy brain who just knows." He took the same information everyone else had and jumped immediately to the incredibly unintuitive, complete-contradiction-to-everyday-life right answer. Arguing that everyone else who tried and failed was 'just as brilliant' is a little like saying that everyone who fails a test was 'just as good' as the ones who passed. -- Well, I won't actually deny that it's not impossible, tests are usually pretty badly designed at actually testing your ability at physics; but you have no evidence of that, the only evidence you do have says that there was a line drawn.
Like, I'm basically going to cite Kahneman here -- if all you know is that someone succeeded at a difficult task, the correct reaction, the only correct reaction, is to update your beliefs in favor of both "she's very good at her job" and "she got lucky" at the same time.
In all fairness, there is a LOT of work that is, one might say, "precursor to Einstein." We talk about
Lorentz invariance and
Lorentz transforms for a reason, because Lorentz did a good deal of the mathematical heavy lifting. General relativity is even more like this, appropriately so because it's a much more complicated theory that even Einstein had to work on for a good long while.
Einstein definitely made huge contributions that leapt things forward... but while he was
a magnificent galaxy-brained world-historical genius, he was very far from the only one of the period, and any of a number of other people could have stepped into the gap left behind if he'd never made his most prominent discoveries.
Not really, no. Actually, basically all of modern physics begins with the word "Ansatz:", i.e. "we pulled an answer out of our arse made an educated guess, and here look it turns out it works!"
All you need to do is prove that it works, to show enough of how your theory works that people can understand it, make predictions, and test them. Being able to retrace the steps you followed to get there isn't nearly as important, and for good reason -- often people don't even know themselves where their ideas come from. Plus, like, a good chunk of the problems we face are literally not soluble by analytic/rigorous means; you have to try and look at the system and make good guesses, there just is no other way. (Some of these problems should be suffixed "no way that we know of"; but some of them shouldn't, are actually provably insoluble in standard functions.)
The thing is, this kind of brilliant masterwork solution typically is the result of someone who is very brilliant
and also armed with the work of other geniuses putting things together in a way that even laymen can understand the importance of. It's a lot easier to explain what Einstein
did with the work of, for example, Ricci and Levi-Civita, than it is to explain the work of Ricci and Levi-Civita.
The actual thought process isn't "I pulled an algorithm out of my butt and hey presto, it works." It's "I spent a very long time combing the literature for mathematical models I thought would accurately describe this situation, and hey presto, I finally found one that fits the situation!"
Another aspect of this that doesn't seem to have come up is that "great" men or women should be kept out of any position where they are doing something that depends on them long term. The problem with a genuinely irreplaceable person is that sooner or later you will need to replace them. The key bit of evidence that Alexander really was a "great man" isn't his conquests; it's that his empire fell apart after he died. "Great Men" make bad long term leaders because the systems they create around themselves tend to fall apart when they leave or die, because normal people can't keep them running. As I once heard it said in a business context, "You don't actually want the 'A Team,', you want the 'B Team' because you can always get another B Team when you need it".
A "Great Man" like Marx or Einstein will actually do better long term since their work is not dependent upon them. When Einstein died the Theory of Relativity didn't suddenly collapse, because it didn't require him to prop it up; unlike what happened to Alexander's empire. A "great" person in such a field in more likely to have a long lasting legacy beyond the aftermath of collapse. But by the same token they are harder to identify as such, because that collapse is the best way to identify a genuine "Great Man".
I think the pivotal difference is between... fuckit, I'm gonna borrow a concept from David Eddings.
Some people are doers of deeds, and some people are performers of tasks. A deed, once done, remains done, and its consequences do not go away. A task, once performed, has to be done over and over to maintain the desired consequences.
Having a Great Person
do your deeds (liberate the country from the foreign invaders, for instance) usually works out pretty well.
Relying on the Great Person to
perform tasks (governing the country after the liberation) does not work out so well.
The problem is that deeds and tasks are often related or symbiotic. Scientific data collection is a task; scientific explanations for unfamiliar phenomena are deeds. Winning a battle is a deed and if you win a battle hard enough under most conditions, you won't need to win another one... but governing a country in the aftermath of winning the battles is a task.
As you say (but in slightly different words), it's disastrous to rely on the Destined Hero to
perform your tasks, because by definition, the task will outlast the Destined Hero's continued ability to fulfill his duties. Relying on the Destined Hero to do some deed that changes everything irreversibly is a different kettle of fish.
As an example, the US badly needed George Washington during the American Revolution, because he was just about the only person available with the charisma, political savvy, and basic-tier military competence required to hold the Continental Army in the field... and the only way to beat the British was to just
hang on. The US badly needed George Washington during the writing of the Constitution, because his role as a reluctant leadership figure helped set durable precedents about how authority was going to work in the new republic. But after that? He stopped being indispensible, because governing the country was now a task, not a deed.
