The TWs were made by Emps, and then discarded in favor of the Primarches and Space Marines. The other powers don't use them, in your argument. We know that fixing them is unlikely and if possible will take a bugfuck long time. Just because we might research Avernite species 'eventually' in the nebulous future does not mean we will do so.
Like, you know it is a longshot with a ~century long fixing process caveat. Like, moving up a tech tree that other sane powers have been developing alternatives to. Callamus' Machine Legions which they are going to continue developing and which we can take advantage of at no cost. The Primaris Marine project being worked on we'll get at no cost if successful. Whatever research the Primarch factions are doing. All major and significant competition to Thunder Warriors. Just look at the Legio Cybernetica option!
You argue that no other power will research Thunder Warriors. Look at it again. It's actually damning condemnation of the TWs. If nobody else researches them, nobody else has a reason to upgrade them. The immensely psychic resistant mass produceable super-elites are expected to provide a +5 against Beta+ Psykers. The Saurus option is expected to provide a +50, +30 if you exclude the hero. The Sirens provided +30 to Alphas. Grandmaster Psyker Hunters provide +10.
Then you look at the benefits of Biologis researches. Blink. Phasing. Nogs. Beergrass. Nid Poison. Potential Nid research benefits. Spiderbane Dragonflies (1% Growth). Reduced casualties with every lethal species researched. Reduced maintenance costs (Poison Lichen). Monster bait (Lily). LEF Deathstrikes. Berserk Gas (See the war on two fronts). Massive potential upgrades - Just look the Illusion Pine option or the Carniflower Fortifications. Contrast this to the long TW option of uncertain but low possibility of providing marginal upgrades to something with major competitors (Space Marine & Variants, Legio Cybernetica, Alkahestry possibility, etc).
Like, yes. It can produce duds. Or relative duds. It can also produce game-changers, like Nogs and Beergrass. Hell, those actually caused relationship changes with the magnitude of the their effects (Making Midgard the Hive World self-sufficient for awhile). Like arguably all the big effects from Biologis aren't combat.
I'm just saying.. please don't bin the possibilities of the Complete Examinations. I know you've gotten annoyed at all the wildlife omakes at times and voter pushing for specific specie, sometimes what they wrote themselves. But even if you restricted yourself to nonomake specie, there are so many options. Options that aren't.. supersoldier upgrades.