In this comment I’ll discuss longevity and long-term planning from the technical side.
ClassicPress versioning works using semver which is an industry standard that sets rules for how software can be upgraded across versions.
The main rule is that a breaking change must come in a new major version.
To put it another way: After ClassicPress v1.0.0 is released, we guarantee that your sites will continue working on any future ClassicPress releases in the v1.x series. This means full compatibility with all plugins and themes that are supported by WP 4.9.x.
We plan to support the ClassicPress v1.x release series with bugfixes and security updates for as long as people need it.
If there are potentially breaking changes in ClassicPress v2.x, we will add a new screen to the upgrade process explaining the changes very clearly and carefully. We will also do as much as possible to automatically confirm that your site can be safely upgraded to the new version.
Upgrades to a new major version which contains potentially breaking changes will always be optional and clearly communicated.
So, that’s our basic plan for backwards compatibility, which is an important part of longevity. You can stay on ClassicPress v1.x (with full plugin and theme compatibility) for as long as you need it.
Another important part of longevity is what will actually go into ClassicPress v2 and future versions.
The community is determining this via the petitions site, and we are listening. For now, the top category of petitions is “remove X feature that I don’t need”.
We can’t just remove features for everyone, as this would break sites. However, we can move things like XML-RPC out into core plugins, which are enabled by default but can be disabled and deleted.
We will be as careful as possible, but this is a major re-architecting of the ClassicPress code, so it still has the potential to break sites.
We agreed in our December 12 committee meeting that based on community feedback so far, the main goal for ClassicPress v2 will be moving older and less-wanted features out into optional core plugins.
The goals and direction for v3 will also be determined by the community. And like Scott and Wade have stated, I plan to continue contributing to ClassicPress for a long time.