I also had 20k dust accumulated a couple of weeks ago (then crafted a couple of cards for wild). I'd say that if you buy both pre-launch bundles it's not that hard to accumulate dust with the current rate of nerfs. The trick is to not disenchant any duplicates and cash in on free dust after a card gets nerfed. For example I had 28 copies of Gibberling, including a golden one, 20+ of First Day of School (again with the golden one), these two alone netted me more than 2500 dust. These plus some golden epics I had (Flesh Giant for example) helped me collect more than 7-8k dust just from the last 2 months of nerfs
Honestly, it's not even too hard to do it without the bundles right now. Like you said, though, the number one thing people screw up is dusting everything when they don't need to.
That said, playing arena is the best way to build a collection f2p, but a lot of people are still super against that.