Personally I don't mind the new characters but don't get me they were unnecessary to the show but I feel like they aren't as relevant of enough for a reason of why the show declined in a quality. It more of a narrative perspective why the show declined in a quality, the original 5 seasons was about Timmy growing up as well on focusing Timmy relationships with other like how he positively or negatively affected them. As a result of that his relationship with them grew stronger like Cosmo, Wanda and Timmy relationship.
We see the growth of Timmy of become a better person in life and eventually one day in the future we see him happy and fully grown-up with his life with his unknown wife (personally see Chloe as the wife ) and children. The original premise was always growing up and that you affected anyone positively life just by your actions, your choices matters. It taught all people a powerful moral in life.
In the newer seasons however it more like what situation Timmy is struck in with magic, because that changed what the original run of the show so great and relatable just cease in the newer seasons. It doesn't mean the newer season are bad necessarily it just feel kinda of hollow without it original premise.