The conclusion that sites installing the Classic Editor won't adopt to Gutenberg has a logical flaw. The Classic Editor is also used for sites, that are not "ready" for Gutenberg because it uses plugins that haven't gone through a full compatibility tests or where the site owner decided not to jump on the first version of WordPress 5.0 and waits until the kinks are out, say until 5.0.2 or 5.0.3. So there might be a lot more installs of the Classic Editor plugin coming, just because of this normal hesitance to jump on a new software feature out of the gate.