Umm, to be fair...you *can* boost to 70. I actually saw a new player who literally did just that, as they wanted to rush into content. They thought that since they were "pros" in WoW that it would be very simple to just boost and be an expert here. lol.
Granted, those are out of the ordinary situations but just wanted to mention for arguments sake. Or could look at me. I did WHM to 74 or so but had been way past MSQ,. so I stopped and didn't want to lv something from scratch. So I did boost warrior from like lv 30-70 and started playing it. Originally lv 70 content felt simple and me OP compared to the 30 content where I didn't have a lot of the mitigation skills and self heals. But then as I got more to 74 and all, I've been a little scared to go in. I think what did it for me was a couple douchebag groups and then another one where I was the one who screwed up on mechanics. A lot of content was kind of a joke but they did increase the difficulty in this newest expansion in regards to mechanics hitting more frequently and all. Once I hit that bit of tankxiety, I boosted Astrologian and made it my main. Been using it quite often and pretty darn good with it.
Anyway, long story just to say it happens but hopefully not frequently.