So have they failed? I am not sure but the following weighs in favor of that IMO:
1) Most of the original clans have been kicked out or destroyed as “ failures
Starting with the Wolverines under Nicky K. Competitive evolution between the 20 original Clans was apparently part of his plan from the beginning (or shortly after).
or tainted”
"Taint" doesn't mean anything except in the Homeworlds. It's not even clear that the Spheroid clans are aware that the Homeworld clans have labeled them as such. It certainly doesn't mean that a Clan society is no longer Clan -- only in the eyes of certain Homeworlders.
2) In their biggest joint clan effort since Klondike the Invasion they failed to achieve their objectives
From 3049-3052, yes, Operation Revival failed to achieve its objectives.
As of 3145-3150, no less than three Clans are knocking on the Republic's door within striking distance of Terra. It appears that the Clans will achieve their ultimate objective, just not on the timetable of Operation Revival.
3) After the truce and great refusal they battered themselves silly
The Jade Falcons and Wolves did, but now they're both at the Republic's doorstep, so the Refusal War didn't mean a whole lot in the long run.
4) After years of neglect/heavy handiness the scientist cast manage to attempt a wide ranging coup
a. The after affects tainted the genepools and stunted technology progress even further
The Society's revolt certainly changed the course of the Homeworld clans, but we don't know how that turns out yet so no judgment can be rendered.
But we do know that the Society had little impact on the Spheroid clans. It's certainly not clear that technological progress or genepools were significantly stunted. The Spheroid Clans are still arguably deploying the most advanced TRO designs circa 3145-3150 and testing the most advanced technology like interface cockpits. And new bloodnames are appearing among the Bears while the Wolves have enough genetic abundance to let the Kerensky bloodline shrink. Touman sizes are at all time highs, indicating no constraints from the trueborn genepool.
5) After the word of blake wars some of the clans abandoned Zell and limited combat all together which would seem to make them no better than what they claimed to be fighting against…
Clan commanders reduced or abandoned zell long before the Jihad, during certain Operation Revival battles, in fact. It's just the reality of warfare that it cannot all be conducted as a gladiatorial tournament within a Circle of Equals. It has more to do with the nature of the Clans' opponents than the Clans themselves. It's arguably a testament to the adaptability of Clan society that those changes were made.
So what do you all think? Have the Clans the descendants of the noble SLDF failed?
The Clans have certainly failed Alex K.'s purpose in removing and preserving the best of the Star League from the Succession Wars. The Clans bear little resemblance to the SLDF by design. They were purposefully developed and written as an alien twist on the SLDF to scare the crap out of the Inner Sphere. They were a failure of Alex K.'s vision from the beginning.
But have the Clans failed Nicky K.'s vision? Warden goals? Crusader goals? General goals of any society to survive, adapt, and thrive?
No, to all those. The Clans meet Nicky K's vision of a competitive, martial society shorn of the old Inner Sphere associations that led to the collapse of the Star League, the Exodus, the Succession Wars and civil war in the Kerensky Cluster, and the Second Exodus. During the Jihad, when the Inner Sphere most needed help, the Clans answered and were critical in removing the Blakist threat, arguably fulfilling the Wardens' promise. It took about a century instead of decade, but the Clans now appear poised to win Terra and fulfill the Crusaders' promise. Moreover, Clan society remains vibrant after a century in the Inner Sphere. It has more planets, population, and resources under its control than at any point in its history. The caste system and artificial breeding programs remain intact and central to Clan society. And Clan culture is winning Spheroid adherents from Lyran tankers now serving in the Wolf touman to Bear trueborns descended from Rasalhagians.
Has Clan society failed? No. In fact, it's thriving more than ever in the current timeline.
Will Clan society be due for a big takedown sometime after the publication of ilClan? Certainly. But that's the nature of the BTech universe.
My 2 Kerenskies... YMMV.