In Bug 41246, it turned out that skins are maintained by volunteers. This is not an option for a popular CMS, especially if they can't be maintained properly.
Removing the Cologne Blue skin completely after its main benefit was removed should be considered.
Skins often also complicate the situation so compatibility wrappers must be written (cf. mw.util.$content) that in turn suffer from other issues (e.g. being undefined or null when accessed too early) which can only be fixed with huge efforts. Another example is that some links in the quickbar in the classic (&useskin=standard) like the move link do not carry IDs.
What people often want are just some features of the skin like a different background color or smaller elements/padding/margin, a fixed sidebar. I know this because I am involved in the community and take the time listening to people's proposals.
https://meta.wikimedia.org/wiki/Turning_off_outdated_skins goes will just remove skins but does not offer options that work without editing personal js or css.
Encouraging people to edit they personal CSS, can for example lead to difficult and strange-to-detect side effects: Consider a user adding invalid CSS to their file; then ResourceLoader merges this CSS with the usergroup CSS and both will fail.