Welcome @DerpFox,
Thank you for your interest in ClassicPress, and your detailed questions about the future. We understand how scary a jump like this may be.
To start, I highly recommend looking through this discussion about the longevity of CP: Longevity for CP
We will be maintaining full compatibility for any plugins that support 4.9.x. Here are some more details from another thread: Switching to Classicpress (large site) - #3 by wadestriebel
Yes, our migration plugin now support WP 5.0. You can find out more details about the migration plugin here.
For version 1 there will be no breaking changes to ClassicPress. Meaning these types of changes (or a full rewrite) would only be taking place in V2. For version 2 we will be using the petitions site to guide us on what features we add and what features get moved out of the core and into a core plugin.
Hopefully those details will help with the doubts you were having 
If you have any other questions or concerns we would be happy to discuss those too!