I have a Thromok the Insatiable deck that I would so turn into a Xenagos, the Reveler deck if I could, but for some reason only the Planeswalkers specifically from the supplementary Commander sets can be your Commander. To make matters worse, even those ones are forced to include an ugly line of awkward rules text just to make this exception.

Why can't it just be that all Planeswalkers are eligible be Commanders? I'm sure there are a few who could prove to be quite problematic—JtMS comes to mind, as well as perhaps Saheeli Rai or even Nissa, Steward of Elements—but you can just update the B&R to deal with that. What arguments are there besides "this is the way it's always been done" against making this change?

Planeswalkers are legendary now so maybe the rule will change soon. Also just talk to the people you play with

Yeah, the thing about Commander is... it's not officially regulated. WotC didn't invent it and they don't really have any way to enforce rules or anything.

There are some noteworthy reasons to not want planeswalkers as commanders.
1) They kind of suck. They're immediate obvious targets for attack that scales negatively with your number of opponents. You're inviting a lot of bad player experiences by enabling them as commanders.
2) They prolong the game. Not only do they serve as a buffer for your health, but you are also strongly incentivized to build your deck so that opponents can't profitably attack your commander.
3) They're kind of boring. Being a collection of sorcery spells, they don't have the same exciting build-around applications that creatures often can have. Most of their decks just end up as value engines since the commander contributes very little in terms of interesting deckbuilding applications.

The first our the gate for banning's would be Sorin Markov Id imagine. If you could build a very defensive strategy with edict like effect that black has a ton of you could quite literally keep folks dead quite shortly. then if they can get out of it then you have the biggest bullseye on your face at the table

Please lord no. They would have to ban at least 4 immediately, and the format would just become a total drag.

There aren't really that many banworthy Planeswalkers. The prime suspects are Dack Fayden for single-red-handedly making Artifacts unplayable, and Tezzeret for making them overpowered.

Dack, Tezz, Sorin Markov are the 3 "easy bans" Trial Gideon (can't lose the game) would probably be on that short list as well. Karn would probably get banned as well, since it would just be "ramp to 7 and then restart the game four times"

Sorin could get the axe just because casuals would hate him, but having it in the command zone isn't really that much bigger of a deal compared to just having it in your deck.
Gideon is mono-white and does nothing to compensate for mono-white's weakness, and you can just kill the Gideons. A Cyclonic Rift murders you if you rely on the emblem to stay alive.
Karn manages to have a color identity that's even worse than mono-white somehow, you can just remove him. He needs three turns to ultimate, do people in your playgroup just twiddle their thumbs?

"Cyclonic rift beats you" isn't an argument I feel is super valid. Cyclonic rift beats most decks and should (in my opinion) be banned. The big difference between Sorin being in your CMD zone v. being in your deck is that you can easily recast it if it dies as a commander.

The argument is obviously not that you lose to Cyclonic Rift, it's that you lose to any means of removing all your Gideons which is a function the second biggest staple in the format serves. You also lose to Hour of Revelation, Magmaquake, Gaze of Granite, Oblivion Stone, Devastation Tide and just having your Planeswalkers attacked to death. If you want to abuse the Gideon emblem I'm pretty sure you can already do that better with Djeru than Gideon of the Trials.

Why four times? Are you assuming knocking a player out each time.

Granted, I'm the kind of scum who would want to trap the table in a karn-based time loop. I'd also try to work out nested Shahrazads if I could. Not that I'd play such a deck often but there are times when "being a supervillain" trumps "winning".

A lot of legendaries also produce bad gameplay or are overpowered as commanders. The gameplay planeswalkers generate is probably largely less interesting on average but i don't think thats a sufficient reason to not support them considering how well they suit the fantasy of the playmode.

I honestly hope it doesn't. Planeswalkers as commanders would require an entirely new philosophy when brewing decks, since they aren't creatures.

Plus walkers are considerably more powerful than creatures anyway.

I'm just glad Sheldon and the RC have said they won't.

Planeswalkers that can immediately minus themselves to death like Garruk, Primal Hunter become abusable when you can recast them from your command zone multiple times a turn

It would be pretty hard to multicast most of those with the commander tax taken into account. You'd need 12 mana to pull a Garruk draw and still have a Garruk, assuming those were your first and second casts of the game. Even in EDH that's not easy to hit outside of Infinite or Very Large combos. Heck, I'd argue it's probably harder to go for mad loops with Garruk than some commanders that currently exist because Food Chain can't eat 'walkers. Is there any 'walker that immediately self-terminates to go mana positive?

Koth and Xenagos could go mana positive in the right board state. They don’t self terminate though. You’d need a sac engine like Claws of Gix.