I think the "deletionist" tendency is one of the biggest problems with Wikipedia. At least it's the main thing that prevents me from making significant contributions. I say tendency, but maybe it really is more of a crusade. Deletion and rejection definitely seem to be the default "predisposition." I've seen a lot of examples of apparently well meaning contributors being pushed away by the need to establish "notability" for a subject and the expectation that all information must be referenced to a fairly limited number of approved reliable sources. These are norms which have been built over a long period of time so it would be incredibly difficult to change them now.

Exactly. It makes it basically impossible to get niche industry/trade information and history onto wikipedia unless it was so newsworthy it's covered everywhere.

That's a feature. Each article requires future attention and adds load.

Most of the important articles were in the first 100,000.