I have no objection to some plugins (the more widely useful ones) making it into the codebase,but as kiwidude points out, I have higher priorities, like working on parts of the core infrastructure that only I maintain.
That said, if any plugin developer feels like having their plugin become builtin, you need to
1) Convince me that it should be builtin
2) Convert it into a patch against the calibre code base.
3) Accept responsibility for maintaining that patch
A huge number of features developed by people other than me have made it into calibre that way
