That's exactly what happens when playing ESO through steam!
I have no idea how to be more clear...
There should be an option to link your steam account to an existing ANET account.
What should happen after linking is that there shouldn't be any separated steam account if you do that.
Any content you had previously paid for through ANET should be available AND (here's the main difference with ANET implementation any why the link is needed), any content you buy THROUGH STEAM, should be applied to said existing account. (Because it's linked).
Let me exemplify this with a numbered storyline. Maybe then I can make myself be understood.
1. Timmy has had GW2 since it came out, and bought every expansion through ANET. Timmy lives outside of america, so all the payment options of steam have been a blessing.
2. Timmy notices that GW2 is now playable through steam. And he hasn't bought the newest expansion because he no longer has access to ANET payment options. He however, has access to steam's payment options.
3. Timmy wants to log in using steam and link the two accounts, so he's able to pay for the new expansion through steam and the purchase being applied to his old ANET account.
4. Timmy is sad, because ANET hasn't provided a way to do this. That means that any purchase of DLC he would make through steam would NOT apply to his old account.
Timmy's story would be different in other games, because other MMOs that are available on steam and that were previously available only outside of steam have implemented some kind of linking functionality.
I'm extremely sorry if I sound pedantic. I really don't know how to make myself understood anymore.
I want to use my old account and buy new stuff from steam because it's easier. I can't. Other games allow me to do it, like ESO. But not GW2.
Edit: Please, if anything I said was unclear, let me know.