-) how hard people would find it to verify the relevant data before their flight.
After all, even when using Jeppesen or whoever as a data source, isn't one supposed to cross-check with the relevant AIP?
None of that is feasible, at all. In planning and in flight, you rely 100% on the databases. Almost nobody ever reads any AIP - except occassionally for airport details like whether it needs Customs PNR.
Then again, drawing a parallel with streetmaps: when I compare the "free" OpenStreetMap with commercial maps widely available on the internet, I find the free data generally more elaborate and more up to date
Probably because they ripped it off high quality (by which I do not mean TomTom which uses databases several years old) commercial products. You can do this if the end result is thinly spread (e.g. open source data) or if you sell it but keep a low profile.
Due to the widespread disgust over tight copyright control of State maps (most of them having been produced at taxpayers' expense) there is a massive amount of underground map distribution. Do a google on Kabouterbond for one of several "movements". Not much aviation data there but all the stuff can be found on torrents anyway, and current too.
This is why a windoze tablet is a good solution for in-flight moving map GPS. You get Oziexplorer and you can get Ozi maps for all of Europe, and ONC maps for most of the rest. If there was an Oziexplorer for the Ipad, that would interest a lot of people, but one could say the same about Jeppview, Flitestar, Navbox, Skydemon, etc.