Even the study behind Harman target very clearly noted that it is not optimal for everyone’s preferences, it’s just what the majority prefer.

So as it is, aiming for it is generally a safe bet for headphone manufacturers, and if you had to create one set of headphones to please the maximum number of people, Harman target it is. But of course, in reality we have to remember that people buying headphones are a diverse bunch, and if everyone else is aiming for Harman target, it actually might be better to aim for a different sound signature to please the people who would prefer some other tuning over Harman.

So, I wouldn’t be too worried about everything becoming the same, there are good market reasons to keep offerings more diverse even if Harman becomes more popular.