I can see where you are coming from, and I do think there will be people who rank the HD650/HD600 over the HE-400S.
I do agree that it does not really scale up too much, but I view that as a plus more than a minus. It will still pick up different characterizations based on your gear, and you do not need to invest in any specific type of equipment to reach its full potential. There are many other headphones that can achieve really solid performance without needing crazy amplifiers to reach their full potential such as the Ethers, and I do really think that is a benefit for us rather than a con. I do see high performance while being very efficient with less picky requirements being the new standard rather than the other way around with the headphone releases by companies.
I would say though that price point does not really correlate well with performance and I do think from my direct comparisons that the HE-400S can compete well with many much more expensive headphones and is an extremely solid option in the mid-tier category. The HE-400S can easily beat out the X2 and K7xx (which are also really solid mid-tier options) in some technical performance aspects as well imo. There are a lot of more expensively priced "mid-tier" options that do not really have the sound signature tuning or technical attributes to compete against solid classics. If you can't find any area where the HE-400S excels over the Sennheisers, but are interested in trying, I would I would recommend doing comparisons specifically for that sense of speed, transient response (attack/decay) and bass tightness.
I would view the HE-400S as a solid mid-tier offering personally. So I would just caution making assumptions on where a headphone sits in performance based on price point. Its sonic performance is extremely extremely competitive for its price point. There are reasons for choosing the HD650 over the HE-400S, but I think extensive comparisons will demonstrate that they both provide very solid performance while subtle nuances that will sway individuals towards one or the other. I personally would be quite happy using any of those options as my main mid-tier.
Cheers!