This reminds me of a similar issue I thought about when I first learned about the efficient market hypothesis (EMH) with respect to the stock market. The similarity is that if EMH were to be true, it dissuades people from seeking investment strategies that produce consistently out-sized returns relative to their risk, which in turn reduces the efficiency of the market. So you don't want too much belief in the EMH - in the same way you dont want too much belief in the Myth of the Lone Genius (MotLG).
It is an interesting problem to tackle: either we believe in EMH and the MotLG too much and dissuade smart people from making amazing discoveries - or - we don't believe it enough and we encourage smart people to waste time on big impossible advances when they could have been making small marginal steps of progress.
I agree with OP that 'is this thing true or false' is not a nuanced enough conversation to have about such a complex topic, because it seems obvious to me that lone geniuses can exist in a world also filled with smart people making small advances. Arguments based on 'how alone' was the lone genius seem arbitrary, because there is no way to agree on a definition of 'how alone' a genius is. I also agree OP's suggested reasons why one might be motivated to argue in a 'true or false' way.
Some follow up points I think are worth talking about, but that I have no current answers to, are:
Does belief in MotLG actually dissuade the people we care about it dissuading? Are potential lone geniuses likely to be affected by MotLG, or are they so naturally stubborn and resistant to the ideas that they have no effect on this type of person anyway? Are there many potentially lone geniuses at the margin who are dissuaded by MotLG?
If belief in MotLG does genuinely dissuade brilliant minds from pursuing novel ideas in a material way, what is the magnitude of this impact?
What are the trade-offs (if any) between encouraging brilliant minds to find novel ideas versus encouraging larger cohorts to work together?
Assuming we are optimising our society for novel ideas/progress (and not being politically correct, etc):
If the magnitude of the impact is high and/or trade-offs are acceptable, how do we approach presenting MotLG and other such ideas in public discourse?
Do we stop teaching MotLG altogether? Or do we teach a more nuanced version of MotLG with relevant counterpoints? What do we emphasize and de-emphasize, etc?
Do we try to identify potential lone geniuses and treat them differently to the general population? If there are many potential lone geniuses at the margin what other ways can we attempt to get the right message (or mix of messages) to the right people?
What is the optimal mix in belief in MotLG across a given population?
How does this optimal mix in belief in MotLG change when the population changes? i.e. is the optimal mix of belief in MotLG different for different nations, where the average characteristics of that nation means that belief/disbelief in MotLG holds that nation back?
This reminds me of a similar issue I thought about when I first learned about the efficient market hypothesis (EMH) with respect to the stock market. The similarity is that if EMH were to be true, it dissuades people from seeking investment strategies that produce consistently out-sized returns relative to their risk, which in turn reduces the efficiency of the market. So you don't want too much belief in the EMH - in the same way you dont want too much belief in the Myth of the Lone Genius (MotLG).
It is an interesting problem to tackle: either we believe in EMH and the MotLG too much and dissuade smart people from making amazing discoveries - or - we don't believe it enough and we encourage smart people to waste time on big impossible advances when they could have been making small marginal steps of progress.
I agree with OP that 'is this thing true or false' is not a nuanced enough conversation to have about such a complex topic, because it seems obvious to me that lone geniuses can exist in a world also filled with smart people making small advances. Arguments based on 'how alone' was the lone genius seem arbitrary, because there is no way to agree on a definition of 'how alone' a genius is. I also agree OP's suggested reasons why one might be motivated to argue in a 'true or false' way.
Some follow up points I think are worth talking about, but that I have no current answers to, are: