Haha, none of these shoes were meant for walking! On average, I break more shoes than most people buy in a year. However, in 2025 I don't think I've had any shoe failures so far (knock on wood). My interest in heels takes two different paths. Besides liking the way they look, I think of them as an athletic challenge, and most of my physical exercise consists of walking in heels. I have walked a documented total of about 4,000 km over the past decade in heels of at least 7 cm, and in recent years, I don't have anything lower than about 9 cm that I walk in. The 12 cm challenge proposed here will be something totally different for me, because now I will be going for sheer height/flexibility rather than endurance. And I'm not 100% sure how I'm going to accomplish that.
As far as shortened tendons, I can really tell when I've slacked off on the foot exercises and calf stretches for several days. I have to be very careful of that because of the nature of my time in heels. On the other hand, I don't live full time in heels, either, because I can't. I think you'll probably be ok with just the slightest attention to stretching your achilles, because you are not in heels full time, all the time. Whoever wishes for shortened tendons (and in real life, that number has to be exceeding rare) is a complete moron.