Your reasoning is correct, if and only if adjunct professor positions are filled as intended. The article mentions that adjunct professors make up over 50 percent of faculty at various colleges and universities. If it is so -- I am not qualified to evaluate -- then your argument falls at the base.
If adjunct professors are over 50 percent of faculty, then the position is being used by universities to hire cheap. This leads to obvious problems for professors but, ironically, in time kills higher education. Higher education requires individuals who are free from the worry of making ends meet, so they can pursue knowledge. They need not be rich, but they can't be counting pennies. In time, a staff made up of strapped professors hurts the teaching quality, destroys the perception of quality at universities and scares students away.
Why do professors need to "pursue knowledge"? What is wrong with having a corp of workers who's sole goal is to transmit knowledge? To avoid status quo bias, do primary school teachers also need to "pursue knowledge"? Since most primary school teachers do no research, should we fire them and replace them with researchers?
Every good teacher you had in your life knew much more than the level at which they taught you. You can't teach effectively if you don't master flawlessly the subject you teach. You achieve that level by going way beyond what students learn.
At the basic education levels, this is easy to achieve. At the University level, you can only guarantee this elastic push by heading into research.
By this logic, algebraists and combinatorialists also can't teach calculus, given that they probably only got as far in calculus (namely 1'st year of grad school) as all the non-tenured, non-research adjuncts.
Yep, they can probably not teach you much about any non-trivial problem in calculus, nor would probably engage you much in current advances being made in field. They would make fantastic teachers for teaching Algebra-I and II.
Some of my worst courses have been Field theorist teaching electromagnetism.
That's a very simple thing to answer. If your sole goal is to transmit knowledge, then you don't know how to teach to create knowledge. And that would be the end of "innovation".
If adjunct professors are over 50 percent of faculty, then the position is being used by universities to hire cheap. This leads to obvious problems for professors but, ironically, in time kills higher education. Higher education requires individuals who are free from the worry of making ends meet, so they can pursue knowledge. They need not be rich, but they can't be counting pennies. In time, a staff made up of strapped professors hurts the teaching quality, destroys the perception of quality at universities and scares students away.