1. As far as I understand, a clean separation was needed to ensure CI4 was feasible and had a future. The incompatibility has been known for a long time.
2. CI4 supports modern PHP versions. Older versions are not maintained or supported by PHP and do not receive security fixes. In the real world I know it's not always possible to have the latest all the time, but using unsupported versions of PHP isn't advised either.
3. Personal preferences, but as you say, it's optional. And using it will helps with (6).
4. Using namespaces will solve the problems you mention in (6).
5. I haven't witnessed this but don't check the CI4 code unless I really need to, and probably wouldn't bother me anyway.
6. See 3+4
There's nothing forcing you to upgrade to CI4 and you're free to continue using CI3, fork CI3, or move to a different framework entirely.
From my perspective, having worked with CI since version 1, I think CI4 is the natural successor to CI3. Even though it's not a straightforward upgrade, the effort to upgrade will be likely be worth it for most existing CI3 projects I maintain.
