IMO the message this dues structure sends is that this organisation is for commercial users of MediaWiki. If you can easily afford $100, good for you. If you are a poor student (or not student; the median annual income in Mexico is $5000, to pick a place at random), who spends their weekend improving MediaWiki, too bad for you. I don't think any group with such a due system can reasonably claim that it represents the entire third-party MediaWiki community.
Which is not inherently bad; more representation for enterprise users would certainly be great, and it's easier (and more credible) to represent a more narrowly defined constituency. (And arguably the group is already almost exclusively focused on enterprise usage.) It just doesn't match the way the group currently presents itself.
Also, it would leave a gap. Again, that's not inherently bad and not something a group can be faulted for, but in that case I'd hope someone steps up to create another user group that represents non-enterprise third-party MediaWiki users.