You're assuming they're working on the DLC one at a time.
Yes, because that's the way they've always done it.
The only time multiple story DLC were in full production at the same time was ME3, because the Omega DLC was being done entirely by Montreal (leaving the actual ME3 team to develop Leviathan and Citadel, sequentially, interspersed with the crap packs and multiplayer expansions, which are handled by a different team, the live team, or sub-teams from the main team who can be spared).
More likely
You say this without any factual basis for the assumption. That's why it's an issue.
I do not understand why people thinking that the next DLC will be the last? Because one posting from one online magazine? the author do not like BioWare/EA, maybe.
Because BioWare conducted a marketing survey that outed the DLC as the final adventure for the Inquisition.
Nobody knows for sure if the next DLC is going to be the one from the survey, but signs are definitely pointing that direction.