Keeping My Opera around would have meant that Opera would have to had allocate resources into it, resources that were (and still are) more needed in the browser itself.
My Opera was not making any money for Opera and only thing they were getting from it was brand loyalty and good will from the My Opera users.
My Opera was a really nice thing to have and I am sad too that it is gone, but I am not faulting the people who were paying for it for closing it down.
Personally, I find Facebook to be untrustworthy company and would never give them my information to sell.
Don't really see Vivaldi as Facebook clone, it is more like clone or child of My Opera. Vivaldi is not perfect (neither was My Opera), but they are working on it.