First let me say that I haven't looked at Trac since I ditched it from production (prior to the Bloodhound incubator). So don't expect this to be fair criticism today.

> It seems not that many people have enough time and long-term interest in contributing significant improvements to such projects.

I am not convinced that is the only reason. The trac mailing list was (in my opinion) hostile to people that showed up with an idea and willingness to work on it; nobody would give encouragement or guidance and there was usually a "fk off and fork" or two in there. People aren't going to stick around to contribute if the maintainers/community are not interested to include them. That is/was a serious shortcoming.

The data model was not well thought out, with lots of UNION and UNION ALL in the queries, row-modelling things that should be normalized, and so on. That's probably my #1 technical issue, in terms of the code, it was hard to correctly add functionality because things were just read into RAM and manipulated there (expert SQL knowledge is of no use when working with the trac database)

The switch of templating engine did not make the codebase simpler, but instead made everything more convoluted and difficult to work with.

The primary issue though, was the lack of Milestone customization, and the incredibly poor performance. I had to implement custom search, custom indexes, custom reports, and custom everything else. And this was a 5-person team, using only ticketing (no repo, no wiki, nothing).