Why ? Of course new players will join the new servers, and i hope for them that they will be able to experience a game without duping, but that doesn't mean that old players on the legacy players will be wiped and have their work gone.
Read the Article again about legacy servers. Only servers without human activity for weeks will be wiped (well, i hope they won't change what they said) so even if there are very few people on the server, it won't be wiped. "the goal will be to preserve any place with human activity." So if you continue to play, your server won't be wiped (i hope)
Without new people, the total population will decrease of course, but real players will stay, some players play almost since the start, why would they stop there ? I think even one year after the release, a lot of people will still be there. I don't think a lot of new players will join the new servers at the release, so some of them will certainly join legacy servers to be able to trade stuff.
And as old players, we won't be left out, we will still have updates, and we will have access to new DLC that will come in the future, i don't think legacy servers will die pretty soon.