In my dojo, we often practice Aikido techniques while holding a sword. It demonstrates how very minor movements can have a big impact. If your body is in the correct position, you slice uke, if you're in the wrong position, you miss.
For example, put a sword in your hand while doing Shihonage. If you let your hip drop back a tiny bit more, at the end of the technique, enough to straighten your elbow pointing your center at Uke, not only does the sword cut off uke's head but shihonage suddenly requires no force on the wrist or shoulder to drop uke. Try it sometime, it's eye opening. There are plenty of others where the sword either cuts uke or misses uke. Anyway...
To me, this type of bokken work has obvious correlations to improving Aikido techniques. But that's very different than standing by yourself cutting air with a wooden stick.
I spent 15 years cutting air with a bokken before I attempted to slice a Tameshigiri mat with a live blade. I had to break all sorts of habits to actually cut it cleanly.
Tenkan is the same thing. You could practice it for years by yourself, but until you feel the 6'5" guy and his two handed grip of death all that practice will likely be thrown out of the window.
I think Aikido requires the uke/nage relationship, otherwise you're just picking fights with yourself. Wow! I think i'll pat myself on the back for that deep thought.