What good is the senju DNA alone? Ability to control wood? That's pretty lousy, in the end. It certainly didn't help Yamato much, in the end.
He was too withered and weak to do anything at all anymore. Who knows, he might have been living in constant pain as well, with his body like that. Honestly, I think offing himself for the time being, which was long years, was quite understandable. It's not like he would have had unlimited chances if Obito had failed, so it was all the same to bet everything on a single card. In fact this way he wouldn't have needed to ever suffer regret in case of a failure; he would have simply remained dead and thus beyond caring.