And if one is going to use these tai chi principles, I really think they should describe their usefulness in how they apply to Aikido technique, not to simply be able to emulate O'Sensei's Performances. If he thought being Unpushable was essential to aikido, he would have taught it to Somebody, doncha think?
He did. for example to Koichi Tohei Sensei who then taught it to quite a lot of people in his own way. Many of whom later due to history did not pass it on.
and I say that from personal experience as an Iaido teacher. I have had iaido students from many styles of aikido, because Iaido is another thing outside of aikido politics. It's made me appreciate the focus on mind and body coordination even when I didn't appreciate the lack vigorous practice sometimes. I am sneaking in internal stuff all the time from Tohei Sensei and elsewhere, because it's universal in having effective technique. Whether it bleeds back into their aikido I don't know.