What would you rather replace - brake pads or transmission parts?
You're not abusing an automatic transmission by downshifting it manually and taking advantage of engine braking, it's designed to do that. You obviously don't want to slam it into first while you're doing 70 headed downhill but gently downshifting is no problem. You are, however, abusing your brakes by riding them instead of downshifting while coming down a grade. That practice will cause you to replace your brake pads as mentioned, but that's only if you're lucky. If you make a habit of it you'll quickly be replacing rotors or maybe even the whole vehicle when you lose the brakes and go rolling off the side of a mountain.
Until they break, which happens more often now with transmissions than 10-15 years ago, because they are so complicated now. They are marvels of modern engineering, but for the most part, they aren't as reliable as the old mechanical four speeds of not so long ago yesteryear.
That couldn't be further from the truth. Modern automatic transmissions are leaps and bounds more reliable than those of 10-15 years ago. They're so good now that manufacturers don't offer manuals on most models. In the past if you wanted to keep a vehicle a long time and didn't want to deal with rebuilding the transmission every 100,000 miles you bought a manual, now that's no longer the case.
