> "I'll create the software X to fit my own specific requirements/needs, then I'll see if anybody else wants to use it as well (which would mean that that niche of people have the same needs/requirements), and if that's the case then I can decide if to put more energe/effort/$ into it otherwise it will just be something useful for myself" > > ..., right?

At a high level, yes.

(At a lower level, well, there will be variations on the same basic idea. The devil is in the detail.)

> I think that for that to work the important detail about the concept&implementation is that the SW must be complete > ... > and that the full functionality will be always available, even if in the worst case I don't have an Internet-connection.

Yes, GigoBooks is intended to be desktop linux/mac/windows software that runs entirely offline. If the OS doesn't change, it can keep operating forever.

> Btw., the GigoBooks app does not currently allow to save data, or did I overlook some menu-entry?

You're looking at the online demo which is necessarily functionality-limited because web browsers don't allow access to local files.

The 'REAL' thing is a desktop app which does allow saving data to local (sqlite) files (and yes, there are additional entries in the menu for this).