Here’s my illustrated argument for lower pledge caps being better. I hope it doesn’t out me as being a terrible Beeminder customer. 
The red line shows the max derailment fee I’ve paid in each year, the blue line is the total I’ve paid Beeminder each year (including the green line which is my subscription fee). The bars show the number of times I derailed each year (right axis) and mostly for my own amusement I’ve split the derailments into which rough category of life the goals were helping with. The most I’ve been stung for is $90, about a year after I started using Beeminder, and all that seems to have done is put me off. I derailed entirely on that particular goal, and though I would have said I got a lot of value out of Beeminder over the next few years it’s clear that I derailed less and paid up less. I mostly made everything safer (though with a non-negligible “I got better at doing things” in there too).
During 2020 I decided to push myself more, skate closer to the edge, risk derailment a lot more, but reduce the sting of each derailment. I created a lot more new goals to try ideas out as well. I still have pledges capped at $30 by default but mostly reduce them down and try to reevaluate whether my targets are reasonable regularly (I should probably beemind that!). I’d definitely say that Beeminder has more value for me this way. And it looks as if I have more value to Beeminder this way as well. I’m not just thinking of the slight rise in fees I paid Beeminder in 2020 compared to 2019 but also that the long tail of the graph is probably sustainable income for Beeminder for years to come rather than trailing away to nothing. I’m a happy repeat customer and hopefully not one who takes up e.g. lots of support resources.
Whilst I understand how the pledge-tripling schedule should motivate me in practice I find the gaps between the amounts too big to climb above the first couple of steps of the ladder. It’s pretty much how one part of my brain understands I’m safe looking through the glass at the top of a tall building, but another part is just “aargh, let’s pretend I’m not here” (which in Beeminder terms is something like “let’s pretend and enter fake data” which then ruins everything).