I'm going to reply here, but I think this covers several parts of the discussion elsewhere in the thread too... :-)
To be clear: I'm ABSOLUTELY NOT saying that "champions do not exist all at the same time". I'm saying that certain stories do not make it clear whether or not specific champions are active in the same place, at the same time.
This works both ways. If we assume there is a "now" (which there is!) and that all ~140 champs are active right now, then they can all potentially interact with one another in whatever ways make for the best and most exciting stories. You can expect to see more and more of that kind of storytelling in the near future.
However... if there is a "now", then there was a "past", and therefore an implied canonical timeline. Riot has told stories set in the past of the timeline - even the Darius comic showed some events from 5 (?) years ago, and as Xenohaz pointed out, Braum has apparently been around for multiple generations. This means either that humans in our setting don't age or die (seems unlikely!) or that we have to assume that some champions are very unlikely to ever interact outside of the game, because the days, weeks, months and years of their lives are gradually being filled up with storylines.
If we tell a story set 5 years in the past, or an origins story for a champion even further back, then we have to consider the fact that younger champions would be children... or not even born yet. Then you have to literally start working out "when is now?" and how old LITERALLY every single champion is, and whether or not they age like regular people.
That's all I meant :-)
There is positive movement at Riot towards making internally consistent lore - we're not there yet, but we're making progress. I'd urge everyone not to see this as a limiting factor, but a chance to see more stories which have meaningful connections and an internal logic across all of them.
And as always, I welcome your thoughts and feedback on this. I wouldn't be here if I didn't want to hear what you all had to say.