What would be the advantage of increasing the limit to 20,000 chars?
With 20,000 chars, and an average of 5.1 chars per English word, that's 3,921 words - about 4 full pages of 8 1/2 x 11 paper.
If teammates were posting 4-page messages to Mattermost, I'd tell them to stop and use attachments.
Just because you can do something doesn't mean that you should 
Personally, I'm glad there's a limit of <1 full page of text for messages.