Ah yes, Xerath agrees. His ties with Runeterra were flimsly indeed, he needed that new motivation and modus operandi.
Trundle evidently couldn't have kept his old character either. No, just toss him away and replace him. Couldn't possibly have a cynical runt for a leader.
And don't worry. Riot won't repeat the past, ever. They won't retcon characters you all love and care about for new ones. That'd be outrageous.
(Yes, this is sarcasm.)
There is evidence that Riot removes characters from the lore. I see no reason to assume this won't continue. It was not a one-off. Trundle got this treatment, but Xeraths modus operandi and main motivation got changed. In addition, I've seen other people argue that Karma and Ashe have been changed as well, though I will not support these statements due to a lack of knowledge.
As it stands, it's most reasonable to assume that Riot is willing to remove or change core parts of characters without community input before or after the fact.
It is also not unreasonable that there will be another premise-shattering retcon: The basis of the current retcon is simply that Riots writers were feeling constricted. Therefore, whenever Riots writers feel constricted, there is a possibility of a retcon.
I'm very unhappy with all of this, but it all seems reasonable and validly argumentated to me. There is no reason to care for these new characters or the new world for me. It might all be gone in three years.