Just because Ripple is centralized makes it becoming very very useless compared to bitcoin, like a huge step backward.
So would a "decentralized Ripple" be a "huge step forward compared to bitcoin" ?
Well... Yeah ?
I mean, Ripple is like all altcoins that are more btc clones, it has plenty of advantages compared to bitcoin. Bitcoin is highly overated now compared to most of alts, so if any of these alt would replace btc it would be a step forward in my opinion. If you add to this the centralized thing to Ripple, the balance is not equal and this disadvantage weigh more than the advantages.
Even if, as you said, people would more use Ripple than NXT (for exemple), which is very possible I trust you, it still does not make it a better coin. This is not the spirit Satoshi wanted to carry with bitcoin, not at all. I don't think he would be glad if coins like Ripple succeed.
Other thing, I don't think touching Wall Street traders is the first aim for a cryptocurrency. In my opinion it should in fact be the lasts, Public awarness will make traders use a cryptocurrency. People would like to buy things easily in a safe way, 99,9% of the world population has absolutly no idea how big things in Wall Street works, including me in fact. It represents propably less than 0.001% of the population (not including peoples who are never going to use any cryptocurrency in this century obviously, same for a computer).
What I wanted to say is that I don't think Wall Street and big marketplaces are a good argument
This is my opinion as a crypto noob, not saying this is the truth, but it seems pretty logic to me. Maybe i'm wrong, I'm just talking friendly here, feel free to correct my talks, I'd be glad !