Uhmm... your post sets up quite a quandary for many of us, here. Your starting position is that you refuse to change your mind and even consider the possibility of trying to shift your 'tiels from an all seed diet to something far healthier and more variegated. But, in doing so, you make it quite difficult for anyone to answer your question without inadvertently co-signing on your decision.
I'm not one to demonize seeds. In moderation, they can even be beneficial. But not when given as the entirety of their diet. Now, from all that I've read, cockatiels are better able to handle an all seed diet than most other parrots.
BUT... feeding them nothing but seeds still renders them far more susceptible to liver disease, fatty tumors and arterial build-up. It also leads to nutritional deficiencies and will almost certainly take away from the potential strength of their immune systems. These factors, and possibly more that I've either forgotten to include or don't yet know myself, can significantly decrease a cockatiel's potential lifespan.
You've said nothing will change your mind, so perhaps what I've said will do nothing to sway you. But I can only hope that you've somewhat overstated the intransigence of your resolve on this. I can understand your frustration. Parrots can be extremely stubborn when it comes to changing their diets. For the most stubborn cases, it takes a considerable investment of time, patience, and even creativity to yield the desired results.
But I'd say our feathered friends are worth it, no?
