During the 1800s it was generally recognised that 'great men' - including geniuses - were essential to the survival, problem-solving ability and progress of societies^. If there was an insufficient supply of geniuses, then society would be static at best, and would crumble and collapse as soon as it encountered a novel threat which tradition or trial and error was incapable of solving.
But through the twentieth century the idea emerged, especially in science, that no individual person made an essential contribution - and that if Professor A had not made his big discovery, then one or several of Professors B, C, or D would have made essentially the same breakthrough within a short space of time. This suggested that science was primarily a process, and that no individual was indispensable.
This idea was propagated even among some geniuses, and even when arguing for the existence of exceptions - for example Paul Dirac (himself a genius) said in praising Einstein for the uniquely personal breakthrough of General Relativity that all other breakthroughs in physics (including his won) merely accelerated the progress of the subject by a few years at most.
But I believe this view was an artefact of the extremely-unusual high prevalence of geniuses in science during the couple of centuries leading up to the mid-twentieth century; the fact that many were working in certain specific areas such as physics, and the sudden pooling of talent resulting from fast international travel and communication. For a while, a short while in fact, just a few decades, there were more physics geniuses than were strictly needed - and any one of them (except probably Einstein) had 'back-up' from one or more individuals of similar ability and interests.
But now that geniuses have dwindled and dwindled in numbers and as a proportion, until it is hard to name any living geniuses in most major areas of human endeavour; I think we are ready to recognise that the usual situation is that there is at most one person in any given time and place who is capable of making a major breakthrough.
And if for whatever reason that individual person does not exist, or fails to make the breakthrough - or if the breakthrough is made but ignored - then that is that: there is no back-up.
If a genius cannot do it, then it is not done.
Part of this is ability - but that is only part of it: the other half is motivation. Most major breakthroughs require several or many years of dedicated and focused work - the kind of dedicated work that can only arise from genuine, spontaneous inner motivation.
For example, for Andrew Wiles to prove Fermat's Last Theorem required not only one of the best mathematicians in the world, but the inner drive to do many years of solitary, dedicated (and career threatening) work - then to fail in the initial attempt, to resume the solitary and focused search, and second time to find the right answer.
Wiles's level of motivation coupled with ability is very, very rare indeed - such that I think we can say that if he had not proved Fermat's Last Theorem twenty years ago, it would still not be proved - and perhaps it never would have been proved.
This, I believe, is the usual situation with geniuses through most of history and most of the time: they do what only they can do; they are irreplaceable.
And the shape of history is substantially affected by the presence, or absence, of such men.
^See William James - Great Men, Great Thoughts and the Environment - 1880. James himself was an example of an irreplaceable Great Man - indeed I believe that the scope of his vital contribution has not even yet been comprehended or implemented.