Anybody who has any interest in the growth of OpenStreetMap has probably read at least one paper or blog that has moaned about "only" a couple of 100'000 users actually having contributed out of the 2 million + that have signed up for an account.
The over 500'000 contributors are a good 25% of the total registered accounts and I'm not sure if that really counts as "only" given that we don't really have any comparative numbers, I would suspect it is actually very good.
In any case there have been calls to simply delete the "inactive" accounts as they inflate the numbers and in general do no good. I'm very much against that for two reasons: on the one hand we don't know why inactive members have joined, maybe they simply wanted to show support, maybe they wanted an OSM account for autentication in uMap or any of many other possible reasons. On the other hand they are a reservoir of new mappers, and every day we likely have dozens of old accounts starting to map.
As an holdover from the licence change I'm still running a script that produces a daily list of old accounts that have newly accepted the contributor terms and typically there are a dozen or so each day. These accounts have not been active since at least April 2011, when we had roughly 350'000 accounts total. The majority tends to be accounts that hadn't previously edited but there are always a couple that somehow didn't get the message during the licence change and had actually edited more than 4 years ago.
In any case the tl;dr version: deleting the 1.5 million inactive accounts would deny us the pleasure and fun of welcoming new contributors like https://www.openstreetmap.org/user/Geocurioius to the active mappers.