Furthermore, those thousands of open issues hint to me that a lot of people are actively contributing, and that the dev team takes responsibility for legitimate problems.
Compare this to other projects with the illusion of quality because most bugs haven't been reported, or because their developers routinely dismiss and close reports of bugs that they don't feel like fixing, just to keep their bug tracker looking clean.
Indeed, the contents of the Matrix/Element/Synapse issue trackers played a significant part in convincing me that the developers are making good decisions, and that investing my time and contacts in this network would also be a good decision. That was over a year ago, and I do not regret it. The project still looks very much on track to me.
As I said before down thread (https://news.ycombinator.com/item?id=27974054), there’s nothing bad about thousands of issues, quite the reverse in fact. Many are feature requests that may or may not be implemented.
A large number of open issues means the project is growing and many people are have ideas for new ways it can improve.