The popular ones are geared for you to rely on them indefinitely; many have "forever" memberships in addition to monthly and annual subscriptions. They perfectly fit the view of Iron Man vs Ultron apps, with the irony that, IMHO, apps in this category should strive to empower users with enough training and motivation for them to carry on their practice after a few months, not compel them to use them “forever”.
Do you have a link to the article? Google Gods did not favour me today.