This was an enjoyable read, thanks for posting, and so rapidly.
You've got a nice set of 'four reasons why people should sign up and stick with Steem', I'm not sure I've got anything to add!
Distribution does indeed seem to be getting gradually broader, which I think is very healthy and thanks for the shout out to @steemflagrewards, they certainly keep me using my DV power!
I've tried to stop posting so much about Steem as well, but I've come back to it a little recently.
I do find this place to be a great way to connect with non mainstream viewpoints, it's been a great 2.5 years I've had here!
As to the final question, I think the only aspect of Steem that's truly immutable now are the raw transactions - basically the date, the type, what other accounts were involved.
No content is immutable - the owner is free to edit at their own leisure, I THINK. Which does offer a rather obvious means for the unscrupulous to abuse the system in the future.
At least with text I get to choose if I want it to disappear, I've heard nightmare stories of people's picture posts only later being available with terrible quality and dtube videos just vanishing.
Anyway, that's just the way it is, I guess we all have to responsible for storing any content we want saved on our own systems.
Nice read, cheers, I look forward to more in the future.