Homebrew or not, RAW allows it. Those are rules. What results from the rules will often be homebrew. Using the combat rules results in a combat unique to the DMs world, making the combat that results from RAW a homebrew.
"This is homebrew"
"No it isn't, these rules allow me to homebrew whatever I want"
I acknowledged that the DM is allowed to homebrew, you acknowledged that it was homebrew. Why does it matter that the DMG gave you a toolkit to make homebrewing easier or not? Especially since I had already recognized that the toolkit in question existed, and since it was specifically for the purposes of homebrew, it didn't count towards a RAW usage of the racial rules.
If I take your argument as you seem to keen on presenting it, then I can play a Selphid by RAW, because a DM could possibly homebrew them. Other RAW races would include Hags, Tarrasques, Dolgaunts, Decepticons, or really anything else, because by RAW a DM could possibly homebrew them, therefore they are a RAW option.
Which, is not what is meant by saying "Rules as Written" since none of those have a written set of official rules for players to use.
For polymorph and Druid shapeshifting, the player controls the contents of the statblock he is using. The DM has no control over it at all.
Wrong. The DM could change anything in that statblock. Raise or lower any of the stats, mess with the HP, alter the AC, all of it.
The player may control their actions, but that does not give them control over the statblock.
Nobody is saying it's the same exact thing. I'm saying that a druid controlling the bear statblock is the same as someone who rolled up a bear, except the druid keeps his mental stats. Using the statblock to play = using the statblock to play.
No it isn't the same.
Centaurs have been large non-fey creatures since 1e(or maybe before) and that didn't change for 5e. Ravnica's change to fey isn't going to be accepted. I wouldn't mind a player playing a small(medium size) centaur, but that doesn't change any of the issues from this thread.
Except for all those issues unique to being large or being over 2,000 lbs.
And it did change in 5e. It changed when they introduced a new type of Centaur. Just like the Minitaur was very different from the normal Centaur.
Also, Ravnica was not the first time that Centuar had been Fey. They were changed from "Monstrous Humanoid" to "Fey" in 4e. In fact, for someone who started the game in 4e, Centaurs have always been Fey, even in the Forgotten Realms.
Were they Fey before 4e? Probably not. They were connected with the Elves and the Forests and all the creatures like Brownies and such, but they were not explicitly Fey to my knowledge, but it doesn't really matter. Changing them to Fey is a trivial excersise of just winding back the clock one edition.
The "problems" don't go away with either medium size or fey. They still can't climb a wall and will have problems getting into a room at the inn due to body size and shape(horse).
Their body size is not an issue at all. Body shape might be an issue, but it depends on what you mean by "issue". I've had "issues" getting a rolling suitcase into a room, but that wasn't more than an annoyance, something trivially hand-waved the same way I have "issues" eating something as bland as gruel. Or the issues of bathroom arrangements or hair care.
And, though I know your response is "but that is silly, it couldn't possibly work!" The Centaur playable race rules do infact say that they can climb. And even if they can't, what, 85% of these issues are solved with some rope, block and tackle, and a leather sling?
Oh wait, now we will add in that it isn't just a wall, but they need to climb a wall stealthily, or they need to climb the wall while being chased by monsters, or they need to climb the wall while being shot by archers, or something else to make it so that that solution won't work.
And I can give you about another dozen solutions to that, and then we'll just go around and around and around. But, remember, the discussion never focused on Centaurs climbing castle walls at night or anything else. It focused on ladders and stairs in an inn. It focused on climbing the rigging in a ship, which hell, lets give that one to that poster, can't can't climb a ship's rigging. Guess what? They can still do plenty of other things on a ship. And none of these things. None of them focused on a time crunch due to combat. So, by adding in that element, you are acknowledging that all of those other problems are solved, because you need the immediacy of combat and threat to make it back into an issue.
Which then opens us up to all sorts of other solutions.