Let's take a step back from the vitriol and look again at what has been said.
The YUI editor: I see nothing to recommend the use of the YUI editor in XenForo. For a start, we build on a jQuery platform, and including the YUI editor would require including YUI, which would add very significant extra weight to pages that contain the editor - which include thread_view - the most commonly-loaded of all pages.
Custom editor: I've said before, and I think convincing arguments have been reiterated in this thread, that there is no appetite or viable reason to write a bespoke editor for XenForo. There are good solutions available under Open Source licenses, and it would be bordering on idiocy to think that a proprietary editor would be as robust in the rapidly-changing browser environment as the big projects our there already.
Ability to replace TinyMCE: Yes, of course TinyMCE is tightly integrated into XenForo - it would not work if it were not so. Replacing any editor with another represents a major challenge, and is not to be undertaken lightly. That does not mean it is inextricably tied into the software with no ability to change it, just that doing so is a lot if work, and must be prioritised against the rest of the huge queue of tasks facing XenForo development.
Why TinyMCE? When we started out, we evaluated a lot of editors for suitability. Of all that we tested, TinyMCE was the best fit. Yes, it has annoying bugs, but the others had more, and in some cases they were show-stoppers. Recently, CKEditor fixed a major bug that was a significant factor in it's rejection from our selection process, so in time we may revisit a TinyMCE / CKEditor swap, but that time is not yet.
