What is the (Long term) strategy for CiviCRM's intergation on CMS?
It's a good question and it has been asked several times over the years. There are pros and cons for both. Up until around v3.0 civi did have a standalone version, and at the time it was felt that supporting that required too much resources, but as you note there's now 5 CMS's and it seems like just as much work.
This post is old but not much has changed except I don't think CiviDesk still offers their version.
For drupal 7 specifically, when drupal 7 itself becomes unsupported civi will probably drop official support for drupal 7, but the community might still unofficially support it for a little longer the same as drupal 6.