Whilst I do agree with many of your points, the failure/breakage rate of these headphones is still incredibly low. They wouldn't be staples if this were not the case.
My DT880's were dropped out of a two storey window by my young nephew while he was left unattended with my audio equipment

(never again) and landed on concrete. They pretty much landed straight on the driver. They were slightly dented, yet despite this, they still worked perfectly.
The appereance of a strong build does not always mean it is so, and vice versa. Headphones are made to be 'flimsy' to reduce weight. However, you have to remember that the headbands of the K712's are made of metal. Despite them being light and plyable, it would take a tremendous amount of force to actually break them. I like great build quality too (think DT1990, Fidelio X2) but with this comes extra weight, and often a compromise on comfort.
Again, if you're looking for good build quality and great sound, you have options at this price range, albeit limited. A tight budget often does not give you great sound, great build and great comfort together - compromises have to be made. If you're looking for the most rugged cans you have the choice of the following -
DT880
DT990 (Too V shaped for you most likely)
Fidelio X2
These are pretty much the only cans under £200 which I would describe as being 'rugged'.
Personally, I find the K712 and HD600 perfectly adequate in regards to build. I am not trying to allude to you being abusive of your headphones, more so that you have simply been 'unlucky'. You have likely been sold 'friday afternoon jobs'. Truth is, typically, you'd really have to abuse these headphones for them to break.
If you up your budget to £300-£350 you have many more options (Sundara, DT1990 etc). But if you're looking at under £180, it is very difficult to get a headphone that are very rugged and come with comfort and sound quality also.
The HD600 is still the headphone I would recommend you, above others. If you're set on build quality above anything else, and don't want Beyer, the obvious choice is the Fidelio X2.