My reason isn't nearly as thought-through as yours! I have my cat because, when my son was little, he and his dad talked me into it. As it turns out they are both gone now and somewhere in there she became MY girl! I'm very thankful that this turned out as it did...she's a great friend, she's beautiful, and she adds great quality to my life.
My second cat came with her daddy...my new husband. But even though she was a "package" deal she has a special place in my heart and I'm happy she's now a part of my home.
It's interesting that you comment about not having to walk your fish. I like dogs as well as cats, but at this point in my life knowing I don't have to take my pets out for walks is a big plus for me! I do find it interesting, however, that you seem to be happy and willing to do the work to keep your tank so wonderfully clean....to me that's at least as much work as walking a pet!! I clicked on your link....your tank is beautiful! And you certainly seem to know a lot about fish!