My position is that the problems exist well in advance of season 8 but that in seasons 5/6 it still felt as if there was enough runway for things to land safely.
This turned out not to be the case. Partially due to creator disinterest (by some accounts, HBO was essentially pushing to give them 10+ seasons with generous budgets) and partially due to a writing style that prioritized "big moments" over character/world building and consistency.
I think this is best exemplified by Arya's "House of Black and White" arc (light spoilers) in season 6. The arc setup is that the Faceless master tells her constantly "To become Faceless you must discard your identity completely -- become no one" while Arya secretly refuses to discard the literal symbol of her identity (her sword, Needle, hidden near the temple). This tension is then essentially just ignored and seemingly forgotten for the entire arc which ends with her being accepted by the master as a Faceless Woman (as "no one") while also maintaining her identity as Arya Stark (by keeping Needle). Nothing in her story ever refutes, upholds, or explores this contradiction. Like many other potentially interesting things, it is brought up superficially (possibly by accident) then lost in the shuffle as the show rushes to the next "big moment".
This turned out not to be the case. Partially due to creator disinterest (by some accounts, HBO was essentially pushing to give them 10+ seasons with generous budgets) and partially due to a writing style that prioritized "big moments" over character/world building and consistency.
I think this is best exemplified by Arya's "House of Black and White" arc (light spoilers) in season 6. The arc setup is that the Faceless master tells her constantly "To become Faceless you must discard your identity completely -- become no one" while Arya secretly refuses to discard the literal symbol of her identity (her sword, Needle, hidden near the temple). This tension is then essentially just ignored and seemingly forgotten for the entire arc which ends with her being accepted by the master as a Faceless Woman (as "no one") while also maintaining her identity as Arya Stark (by keeping Needle). Nothing in her story ever refutes, upholds, or explores this contradiction. Like many other potentially interesting things, it is brought up superficially (possibly by accident) then lost in the shuffle as the show rushes to the next "big moment".