It's simple math: Apple is one company. Unless iPads were so good that they could cure cancer all by themselves, they were going to lose some market share as more competitors entered the field and more quality devices entered the mix.
Even if Apple decided to chase after the low end, there was always going to be a company that could produce something cheaper. An Amazon Kindle Fire HD or ASUS MeMo Pad HD is expensive next to some $50 or $60 slab from a small Chinese manufacturer. And companies like Samsung are willing to flood the market with tablets that cover a lot of niches.
I don't know that the situation will exactly mirror that of the smartphone market, because there are different dynamics at work. Many people see a phone as a necessity, and a tablet as a luxury. That's changing, but it still means that the tablet business isn't quite so price-sensitive. Also, other OEMs haven't quite figured things out... even Samsung. The Tab Pro and Note Pro lines are generally good, but the 12.2-inch models are of questionable value, and the Tab 3 line hasn't been a good value since it was new.