I’m just trying to get to the bottom of the issue. Presumably manufacturers and retailers are in the business of selling headphones and consumers are in the business of buying headphones. The manufacturers and retailers presumably know how many of each product at each price point in each market segment they sell. Ultimately that is the measure that matters. Presumably at a certain price point people try what they are buying and the question is what headphones are people most willing to buy and to what extent does that correlate to the harman curve. The harman curve isn’t a secret to producers so presumably they know how relevant the harman curve is to their most important objective measure: Sales.
You give people too much credit in buying headphones that actually sound good. At higher price points people are probably basing it on looks, marketing, countless bogus reviews that don't use measurements as a backbone. That doesn't make the Harman work null and void, just you have to compare headphones in one listening session with the same varied material to know which you truly prefer and that was done during the Harman work as part of the process, and people at home generally don't do that because it's a lot of hassle, you have to have multiple headphones in front of you & you have to have the time & will to do it. I don't think you can go on pure sales volumes as an indicator of how good a headphone really is. Having said that with ASR gaining popularity in recent years & with people like Oratory & other areas of the internet that measure & review headphones properly then there is some consensus being built on what are good sounding headphones to most people & then that information probably gets out into the mainstream a bit in terms of sales when people are doing internet research on which headphone to buy. So you might find that for recent decently priced (not prohibitively expensive), I don't know say up to £200 you might find that sales figures are being influenced by recent trends in objectively measuring headphones for good sound (ASR, etc) - so I think I'd probably take example of HD560s as the headphone in my mind that has probably benefitted the most by this in terms of sales. I mean I don't know this because I don't have the data, but I think there's a logic to it. I would say the HD560s is an extremely good seller, and yet it's also a good sounding headphone as backed up by measurements. So I think there probably has been some recent trends for moderately priced headphones up to £200 I'd estimate where objectively determined good sounding (close to Harman curve or balanced around it) could have become more popular in recent years and may be a correlation with increased sales. I don't think it affects sales of very expensive headphones though, as people just buy them for "stupid" reasons mostly, however I must say companies like DCA that follow the science on it re Harman or close to similar are very expensive but I'd imagine also very good so it's not "stupid" to buy one of those.
EDIT: and sales volume will also not be an indicator of good sound for low priced products, because it's not an important product to them and they're making quick off the cuff decisions, probably just basing it on the price alone, and looks, & general easy to understand features, and perhaps Amazon customer review stars.
EDIT #2: did some quick research on Amazon just by using search word "headphones" and listing for "bestseller", the HD560s I talked about is listed on page 11 of 20 (see following pic) and by going through earlier pages it's the most expensive product up until that point with the exception of Apple Airpods, some specific noise cancelling headphones, a few bone conduction headphones & well marketed gamer headphones - so to me it looks like a lot of people are buying the HD560s even though it's relatively expensive (for the regular person just happening to buy a headphone) and isn't really an obviously marketed product, so this leads me to believe a lot of people buying this are people doing proper research & finding sites like ASR, Oratory, headphones.com where they measure too - so in this moderate price bracket I think good objectively measured sound through sites like ASR, etc are having an impact.
Note: it's normally more expensively priced than £99