youve said what a lot of us are trying to say,and used experience to back it up.i just added to your rep.Given that everybody has their own opinion, I still think a 5g is the best for a betta. My new betta is 20 times happier and more active in his 5g than the one I had for 6 years in a 1.5g. Obviously there is no effect on life span, but on the overall quality of the fish's life.
I would just not put any fish in there and stick with inverts like shrimp and snails.