As a user, I don't really get the argument of either side.
Apple's killing WPAs because they can't allow 3rd party web engines to do that... That doesn't make sense to me because Apple is still going to allow these browsers to open these WPAs as normal websites anyway.
Now, how is it an attack to the openness of the web, though? Maybe these websites lose convenient access to users? But what do the users lose here? Especially given that they can distribute their apps through third party app stores. I don't really understand the problem.