I’m glad to see that the original point on default licenses that @cazort raised is being discussed as well as various other related issues.
The following information lists the proportion of licenses used by each observer may give us some additional background.
CC0 0.13%
CC-BY 0.23%
CC-BY-SA 0.07%
CC-BY-NC 46.72%
CC-BY-ND 0.02%
CC-BY-NC-SA 0.14%
CC-BY-NC-ND 0.12%
All rights reserved 52.56%
These figures are from a snapshot taken today and show little change from the October 2019 results used on a poster produced by @andrawaag. From this, it is clear that very few (0.71%) observers have chosen a license beyond the initial binary choice offered for their images (All rights reserved or CC-BY-NC).
The good news for me in all this is that while less than one half of one percent of all users have selected a Wikimedia compatible license, they are over-represented when measured by the number of their observations, comprising 6.6% of all observations. This in turn represents a not insubstantial 3.6 million observations. Plenty of material still to go through for those wishing to help Wikimedia projects. More relevant to me perhaps would be to calculate the number of species (taxa really) that currently lack images on Wikimedia Commons, but only exist on iNaturalist from observations without an shareable license. There are over 310k species (more probably taxa) represented on iNaturalist but only between 90,156 (the number of species from observations labelled as CC-BY) to 208,048 (CC0, CC-BY, and CC-BY-SA combined) available for use. As these categories overlap, it is probably closer to 100k that are shareable, with another 200k species that are not shareable under their current license.
Although I have contacted several users to request they change their licence for specific images and many (most) have agreed, as noted above this is a time consuming business that is further complicated by explaining the license changing procedure and the difference between the image licence and the observation license. Straying even further from the topic at hand, I have started a list of taxa that only have non-free images available on iNaturalist and posted the list to an iNat wiki page here. If anyone has images on this list that they wish to change the license on, please annotate the list and I can upload them to Wikipedia.