Yeah, this was really shocking to me. I routinely use the source code of software I rely on to identify and fix issues in how I'm using it. For my day job, I use an "open core" product, and I encounetered a serious performance issue in mid-August, so I dove into the source to debug, until I found the issue was in one of the closed modules it used. In my discussion with support, I mentioned this, and they confirmed: yeah, if it were open, you could debug it, but it's not, so you have to go through us. The idea that no one cares if software is open source or not seems wildly inaccurate through that lens.
Depends on the device...
Symbian^3 was moved to a free and open licence (Eclipse Public License) for some time. The ecosystem died anyway.
Yes, but as a programmer if you had the source code and the inclination you could still make modifications to suit your needs.
I was a programmer at that time and I was even reasonably well-versed in the rather arcane dialect of C++ that was used in Symbian programming.
Making a non-trivial change to an OS and deploying it successfully to a device on my own would be a challenge, though. Once Nokia stopped paying the internal developers and Symbian programming forum was closed, there were no gurus capable of answering questions.
And in case of a very complicated OS whose internal workings were very different from other OSes out there, having no vibrant community of experts = death, unfortunately.
What a horribly short sighted view...
What happens when the company stops developing the software for your device?