So, Blizzard wants these monthly events to keep people busy for the entire content drought until Legion, and beyond.
Two mounts, both reused old assets, each taking roughly 5 months with event quest and a reasonable ~15 runs for each available week. Can't play during one event? Tough luck, it's gonna take you even longer.
How is this enough to keep the average player interested? With such a long wait, the rewards become meaningless. When they could have been exciting if you could just sub and get one of them now, they are just off-putting. Even I, an avid mount collector who stubbornly pushes through grinds, think this is way, way, way too much.
It's so unreasonable that people are just not going to care. "Screw it, if it's gonna take that long it might as well take longer. I'll get it anyway while I sub for better content." Or, "I was willing to give the game a chance for this dangling carrot, but I know I won't stay subbed for 5 more months, so I'm not getting it anyway. Pass on WoW." This is what the average players will say.
So, if you came up with this system, and you want it to be long-lived, why not dig deeper into the sack? Let people earn enough tokens each month for roughly one mount, and simply add a lot more of them! People will not farm 5 months for a single mount, but they might farm 5 months for 5! That sounds more fair, doesn't it?
There are a lot of unused or not implemented mount models already in the game files, and let's face it, those models, even the absolute best of them, are showing their age. When are you going to end up using them? And there is very little cost of adding them, all the models already exist!
Here's a list for more possible Timewalker mounts for several expansions, taken from http://www.warcraftmounts.com/:
The Burning Crusade:
Brown Amani War Bear
Green Amani War Bear
Black Hellfire Nether Drake
Fancy Green Hawkstrider
Toshely's Vintage Mechanostrider
Ethereal Nether Drake
Reins of Ironjaw
Blood Knight's Ghostlands Risen Steed
Warden's Striped Black Saber
Feltalon of the Burning Crusade
Wrath of the Lich King:
Faithful Steed of Lordaeron
Grizzly Hills Wild Stallion
Knight's War Horse
Ebon Blade Deathcharger
Unholy Steed of Naxxramas
Aimee's Taffy Mechanostrider
Darkfallen Black Dragonhawk
Kel'Thuzad's Old Flying Rug
Ghost of Northrend
Reins of the Bleached Zul'Drak Raptor
Reins of the Mouldy Zul'Drak Raptor
Bolvar's Spirited War Steed (Alliance only)
Saurfang's Snarling War Wolf (Horde only)
And, because we'll get them eventually:
Millhouse's Mighty Spellweave Carpet of Doom
Poisonous Tol'vir Scorpid
Reins of the Stranglethorn Tiger
Spotted Darkshore Nightsaber
Firefighter's Aqua Hippogryph
Feralas Hippogryph (Shandris' mount of choice)
Barrens Tallstrider (It can chat, too!)
Magatha's Plain Old Kodo
Kajamite-Powered Mechanostrider Replica (Twice as prone to explode!)
Reins of the Red Twilight Drake
Reins of the Purple Twilight Drake
Reins of the Shy Golden Seahorse
Ashenvale Low-Emissions Shredder
Reins of the First Dragonmaw Proto-drake
Mists of Pandaria:
Unruly Blue Goat
Reins of the Pink Riding Crane
Reins of the Jade Riding Crane
Invader's Armored Skyscreamer
Jungle Armored Skyscreamer *
Kota Kon's Kafa Press
Grand Thundering Dragon Turtle
Effigy of Yu'lon
Thundering Ivory Cloud Serpent
Thundering Icy Cloud Serpent
Golden Pandaren Kite
Grand Pandaren Kite
Mogu'shan Flying Cloud
Stormy Flying Cloud
Green Primal Direhorn
Blue Mushan Beast
Guo-Lai War Quilen
Tomb Guardian of Lei'Shen
Steed of Iron Quon
Blackfuse Faulty Prototype
Kyparite Powered Juggernaut
Black Riding Yak
Blonde Riding Yak
*Leaving the other Skyscreamers for future Zandalari content.
And whow, that's a lot of mounts! All Blizzard has to do is implement them and add them to the vendors, and then, even if players aren't interested in farming them all, it's certain there will be something here for everybody's liking. Even if they're all reskins, and more than half are ancient.
Edit: Added in the Yaks.