If done at least semi-officially it could equally be a way to bring back lapsed players as much as any splitting of user base. Many leave in major updates or exciting new releases. WoW lost loads since LK. In other games LotRO horse riding update lost 30% of the users. GW2 redesigned for the intellectually challenged. I'm sure some would want to come back to the game before it was wrecked. Maybe not so much GW2 - it was always too easy to hold interest for long.

The problem I see is old versions would be unlikely to get updates like new content or dungeons so even those hoping to play long term would get bored eventually.