The unfinished quality you're describing as a plagal cadence is a half cadence, possibly a deceptive cadence, although you are right about that transition forming a plagal cadence. There is definitely finality to a plagal cadence, hence the name "Amen" which concludes a prayer. Also, in traditional harmony a perfect cadence V (5) can't be a minor quality chord (a v). A few not so prominent musicologists/theoreticians believe in a "minor dominant," but that would be included in naming the cadence even for them. So, that Dm7 -G is a very different thing than a perfect cadence. A real snob would also have a problem with you calling them cadences, but for your point it is the best way to describe the importance of transition. Your overall point is compelling, but those details are important to note so folks don't learn these terms wrong.