The sad truth is that when Devs earn a lot, they deliver little and when they earn little, they deliver a lot. This is called greed, they get lazy and roll around for the patreons to continue helping, I've seen this happen with all "famous" games within the forum, the only way to reverse this is to stop helping so he can see the numbers dropping fast and start working faster, the table showing the update time only contributes what I think. 2/2/2/2/4/4/4/6/7/6 (at the moment)
You analysis is fundamentally flawed in that:
1) It takes no account of the fact that, as the game grows, the complexity of the coding does as well so each successive update was always likely to take longer.
2) You make no allowance for the fact that the earlier updates were significantly smaller then the updates which followed them and would therefore take longer to produce each update.
If you were going to make the argument you want to make you need to give a breakdown of the number of scenes/images in each update to the number of months for each update.
to simply say it takes longer for each update when the update size has noticeably and significantly grown means very little and doesn't prove anything. The first updates were very small.
that, in my opinion, is to be expected. Games with a low update rate at the start normally fail to get any backing as people are reluctant to back just off one or two updates as there is no sign of commitment and people have been burned a lot by incomplete games.
Once Devs show that they are committed to the projects and have a backing a Patreons updates then tend to get bigger and slower. this works out okay for the devs who express that this is what they are going to do and not so well for those who lack communication as people then think the work has stopped and bail.
Uber has been upfront that the updates were going to get larger and less frequent and also had pretty good (by the standards of the community) communication with those that back him on patreon and discord about what is going on. As a result his general number of patreons is holding steady around 4000-5000 per month and has done now for the past 6 months.
Also, there is still a real incentive for him to release, not just because otherwise people will stop backing, but also the exact opposite. Looking back over the last 3 years, every update has been met with a significant increase in his backers, most of whom then stick around for the next one.