>We can't say "Just look for the person that's the same in the other world": equivalent persons won't be the same in all respects, since in general they'll have different welfare levels.
Askell extensively argues for why you should be able to do that in the first part of her thesis. For one thing, it's highly implausible to say that differing welfare levels alone necessarily imply an alteration in personal identity. It seems obvious that my life could have been happier or sadder, at least to some extent, without my being a different person. For another, your condition for transworld identity more broadly is way too strong: no philosopher that I know of thinks that I need to be the same in every respect in some other world in order for the person in that other world to be "me."
Pareto explicitly says that you have to keep identities intact, because the definition stipulates that w1 and w2 "contain the same people." If you don't preserve identities, you can't verify that that condition is met, in which case Pareto isn't applicable.