Sure, one executable is nice, that might be enough of a payoff for some people to overlook the rest. I think you may be underestimating the amount of choice that's out there though. For documentation alone there's countless options that don't require a centralized online thing. Three I'd use over Fossil again are 1) simple project doc/ folder with .md files inside (which you can render locally to look just like github with https://github.com/joeyespo/grip) 2) Doxygen or a language-specific equivalent if it's better 3) libreoffice docs stored either in the same repo or another one (perhaps a submodule).

For project management I'm less familiar with the options out there but I'd be surprised if there was nothing that gives a really stellar offline experience. I'd give a preemptive win to Fossil on the narrow aspect that your issue tracking changes can be synced and merged automatically with a collaborative server when you come back online, whereas if you stood up your own instance of Trac for instance I'm not sure if they have any support for syncing. If you're working by yourself, though, then there's no problem, Trac and many others work just like Fossil and stand up a local server (or are dedicated standalone programs) and work the same whether you're offline or online. But when I'm working solo I prefer low-tech over anything that resembles Jira (and I don't even really dislike Jira) -- I've played with https://github.com/dspinellis/git-issue as another offline/off-platform option but in my most recent ongoing solo project I'm quite happy with a super low-tech issues text file that has entries like (easy to make with https://github.com/dhruvasagar/vim-table-mode)

    +--------------+
    | Add thing    |
    +==============+
    | Done whens / |
    | other info   |
    +--------------+

and when I'm closing one I just move it to the issues-closed file as part of the closing commit. I might give it an identifier if I need to reference it in the code/over multiple commits.