Knowing a little more on the ASIC side, I'll say that your thought process is going in the right direction.
The comment about ASICs being more "proven" or containing less bugs is simply a factor of the amount of verification that goes into developing an ASIC. Just think, if it costed $200,000 and took 3 months to print a document, you will likely proof read it (test it) more than if it cost 10cents and took an hour. Through that rigorous process you will probably find that missing comma that if you read the doc quickly, you might look over.
Also if this was your life (and money), you would likely invest 500k or a million dollars into EDA tools & models that allow you to verify timing, manufacturability, etc, etc. all to improve your success rate.
In the end, there is nothing about ASICs that make them intrinsically less error prone. In fact the is probably more that can go wrong.
Fpgas lasting and biggest problem is that there is so much resources that make them field programmable. It's not the logic itself that makes them so expensive but more so the re-programmable routing fabric that makes the programmable. This same fabric is what causes speed issues, power issues and cost issues.
It's simple math to determine what makes the most sense economically for a product. If the volume high enough then the per unit costs outweigh the upfront costs. This is a better fit for an ASIC.
Low volume may be able to bear a higher unit price that's a better fit for FPGAs.
Aside from a pure cost analysis, the bitcoin guys jumped to ASICs so quickly because the algorithm was well defined (low risk of change) and they really needed performance, ASICs will always have better performance than FPGAs because there isn't a programmable routing fabric. Also there is a power problem with bit coins since a key metric is hashes per watt. Lastly if ASICs have lower per unit costs, they are higher margin for the vendors and/or low cost to consumers.
Wrt to obsolescence and tool support, IMO it isn't as bad as the Dave's made it out to be. Sure Xilinx ditched a lot of their product lines when they came out with vivado but that is a little of an anomaly Altera still supports Cyclone I devices which are probably 10 years old now. You can always load old software if you must fix one of those products but it's not an everyday occurrence.
Developing an open source FPGA would be a massive undertaking and would yield something so far from state of the art that I doubt many people would be interested. I don't thing the field is that closed, you can get Altera and Xilinx tools for free and kits for under $100. That's not too bad, it's taken the big 2 many years to get to that point.
In the end though with so many cheap micros coming to market with basically every permutation on peripherals, I think Faves main point is that for any given application, you could probably make a micro or dsp do the job. Kinda sad.
But know that for those who design micros and most ASICs, they are prototypes on an FPGA first. Ever see this:
http://chipdesignmag.com/display.php?articleId=333Before Zynq was taped out it was prototyped on a board with multiple top spec FPGAs