When I was a kid I used to watch Dragon Ball. As you all know, some years ago we couldn't just decide to watch something from the first episode - we had to watch the episode that was airing. And as you may be aware, Dragon Ball (and Z and GT) is full of flashbacks depicting characters from the past and it is also filled with characters from the past who randomly appear. I don't really understand how people can think that the fact that characters appear will cause younger viewers to stop watching the anime, as they aren't fully aware of who this characters are: when those old characters appeared while I was younger, even though I was totally unaware of who they were, I wouldn't just stop watching it - I would get curious to know who they are and what is their backstory. Which is: I would try to watch older episodes. And the Pokémon Industry doesn't want people to forget that there are older episodes - there are re-runs every week.
I think it really comes down to HOW those flashbacks/reappearances are handled: those scenes must be good enough to make old fans happy and new viewers curious about the characters. I don't think any new viewer got curious about Dawn, because her reappearance was pretty boring, but maybe some people got curious about May or Jessiebelle or Ash's old Pokémon when they appeared during Sinnoh.