Hm, Decent Monkey is doing market research.
In my personal opinion, the game is losing patrons for these reasons:
1. The game is huge, and the best content is in the last third of the game. So new people start playing the game, see a lot of grind and average visuals in the first episode and most of the second one, and give up. As a result, there is very little inflow of new patrons, and old ones sooner or later stop supporting because they become interested in something else, get too busy to play games, or, well, even die. New people start playing, see the game from 2018, and don't support it.
2. The business model that Decent Monkey chose for his Patreon account is basically buying each new update (and for a lot of money), but this strategy is constantly undermined by pirates publishing new updates here. It is not that popular strategy with other developers, where patrons are "supporting a creative person to make him able to concentrate on his passion of developing games". Those developers are in constant contact with his or her patrons, listening to their ideas, discussing the development process, making polls on the content, events, etc. That business model is much more resistant to pirates since patrons are getting something else from their support in addition to just downloading a new update, which can be done on any pirate site.
3. Patreon has been getting more competitive all the time. Back in the day, there were very few good games and even fewer consistent developers. There is about the same amount of money on Patreon (maybe less because of the recession, the war, etc.), but now it is divided among more developers, and so Decent Monkey now has a smaller share of that pie.
4. There were a few decisions made during the development of the game that some people didn't like. Cutting out Bardie, getting rid of the Farm plot, etc.