"Ruined the series" I think somebody are taking it a little bit too seriously it was a fun little Easter egg to grow in Evan Peters as Quicksilver nothing more the fans are the ones who went wild with speculation and theories and then ultimately set themselves up for disappointment.
Now did they add Evan Peters to the cast in order to spark discussions? Of course. Marvel isn't stupid word of mouth is free publicity.
But the Evan Peters character was never a central character in the show. So not sure how him not being Quicksilver "ruined" the entirety of Wanda's journey.