Is this because the Wordpress market is huge (almost every website)? Whereas Elastic, Mongo, etc market share is not as huge?

Wordpress is huge precisely because it was open. Elastic is already quite large and would keep growing massively if they stay non-restrictive. Now that they've switched licenses, this may have a significant enough effect over the long-term that they're leaving a lot of "opportunity pie" on the table.

That's great for Matt, but other companies like Elastic can't afford to go from >50% to 5% of their market.

Well, now organizations like Wikimedia are going to have to drop Elasticsearch everywhere despite it powering wikipedia article search and more. So congrats on both missing the GP’s point and also failing to see how this license change would shrink Elastic’s usage / market anyway

They don't need to drop. They're not providing Elasticsearch and Kibana as a paid service to others. You can just read the FAQ: https://www.elastic.co/pricing/faq/licensing Using Elasticsearch and Kibana is still free, unless you're selling them as a service like AWS Elasticsearch does.

People will just switch to OpenDistro.