When it first started, there were great swaths of stuff that could be written. Some huge fraction of history, art, science, etc now all have pages with some level of detail. To continue ongoing maintenance for current events is going to take significantly less activity.

And the stuff that isn’t already on Wikipedia generally has too few primary sources to be allowed to be created. So the ongoing edits would be primarily updates to existing content rather than new stuff.

Has too few secondary sources, you mean. As they say, "Wikipedia is a tertiary source".

(It doesn't help that Wikipedians abuse the "reliable sources" policy constantly to advance their own political agendas.)

I think it was declining, but somewhat stabilized around 2015.