After some weeks of testing with TinyMCE, FCKEditor and Xinha I am sure that any one of them really can replace htmlarea in a fully functional way. Just tested Xinha again and it can even use the same editor functions as htmlarea - basicly you only need to copy all files of latest Xinha to the same folder as htmlarea and create a proper combination of files XinhaCore.js and htmlarea.php and save the result as htmlarea.php (image & link plugins need to use plugin structure of Xinha if possible...)
But... TinyMCE,FCKEditor and Xinha are ALL great editors with some unique features. The only difference between one-editor-system and selectable editor system is that we need to have different files for integration and administration of each editor (only Xinha can use the same configuration as htmlarea - yet htmlarea has less buttons and plugins than Xinha. What I would like to do is to change editor control in administration menu as much as possible "editor independent" and to change main part of editor style administration to theme files and editor files. It would give free hands to develop different versions of editor - role based, activity based, theme based or even user based editors. It would make changing default editor easier - any time. It would make testing new features easier...
All the previous moodle hacks of htmlarea are possible to use in other editors - editable smiley list, hiding buttons, toggling editor... - with small changes to editor code. Upgrading of editor files to latest versions is also possible without overriding any settings or any editor integration files.
Why should we actually remain one-editor-system???