I sometimes will continue to stand if I only have one or two more stops to go. You certainly weren't rude to sit down, if I were in the same situation, I might do the same. But I don't think it was really necessary to announce your intentions. If there are empty seats and no one is moving towards one, just sit down. People may have thought it was odd that you made the announcement before you sat down - almost like they were doing something wrong by *not* sitting down.
In my opinion, the people who get first dibs on the seats are the people who need them. Or who might benefit the most from them.
Of course, it's not really clear who qualifies for that. For me, sometimes the only thing that I know for *sure* is that *I* don't need the seat. So I don't take it.
(as for "benefit the most"--I think people who are getting off last will get the most benefit from them, as will those who are sort of tired, or have a minor hernia, or whatever. They know who they are, and I expect them to look out for themselves.)
And I hope that those people who either need them or will benefit from them will go ahead and sit down.
I think you were in the mildest group--those whose benefit from getting a seat is that they'd prefer to. And you waited as well, much the way I would. But once no one was indicating that their need was greater (they'd communicate this to you all by taking a seat, for heaven's sake, instead of waiting around), your actions were right on the money.
In fact, by mentioning that you were going to take the seat, you gave permission for everybody else to do so. Sort of like, I "serve the world" by being willing to go first in the buffet line if people are hesitant to start. What you did was be a leader, and other people sometimes disparage that. But that doesn't make it wrong.
And those people who were irritated--pah. Who cares? They were rude to express their irritation. If they didn't want to move, they could have slid over.