As far as I'm aware most "audiophile" level headphone cables are intentionally short because owners don't want them getting tangled and being in the way - excess cable is always worse than lack of length due to the fact that, as said, you can just get an extension cord to achieve the desired effect. If it is stated there's only one cord coming with the headphones and that it is detachable I instantly assume it's less than 2 meters long unless otherwise specified, even more so seeing as the 400S are meant to be transportable (I wouldn't wanna walk around with an open pair of planars on my head but point still stands). 1.5 is plenty for me, anywhere between that and 2m is perfect for how I use headphones.
It does also state very clearly on the
Hifiman website 400S page that the cable is 1.5m long as can be seen under specifications, if length is such a deal breaker to you to the point of it becoming unusable then it could've been avoided by being a bit more thorough.
Please take no offence to what I'm saying or as some snide "your own fault" thing, I just figure it's nice to be more informed for potential future purchases.