There is no proper justification for removing Trundle other than that it has been done.
Riot does not care about their old lore and their old characters, they'll happily retcon everything they dislike. They don't care that some of us do care.
They're making a new world with new characters. They'll do this their way and then show us the result, assuming we'll like it. And there will be people that like it.
Issue is, in five years they might just abandon their new world again, unfinished as it will be.
Riot has lost my trust, has done nothing to regain it. They've had ample time to realise they've made a mistake, admit it and to fix it, or announce they're fixing it otherwise. I've stopped caring about Riot and their lore as a result.
Any investment I'd make in Riots lore, of time or emotions, would just be thrown away carelessly by Riot. Because that's exactly what they've done already. Twice.