It's a good suggestion. $139 is some significant sticker shock if you want all the features, especially if you have multiple accounts (either personally or in your household). Or letting us pay with kronos would be cool too, throw 8 kronos at it (approx 18 bucks x 8).
If I understand correctly, the only real difference you get by upgrading to the $139 package is a mount with somewhat better stats than other mounts. It's not for me.
The problem with something like this, what happens if someone plays 3 months after release and is on a month 2 month sub, then cancels and doesnt play? They got to play the content, utilize the rewards of premium, and still owe for premium, but no longer pay to play? One flaw I do see with that method and would create issues as a result. Also in theory if that was allowed, they would do what most financing plans do. Charge interest for the total duration its financed. So if the premium was $139, or 11.58 a month if you put it away yourself, you can bet your butt they would charge $16+ to recoup potential losses if one did stop paying to play part way through. Good idea in theory from a consumer stand point, terrible business practise imo.
I'm sure that if they consider this business proposition they'll weigh the benefits of getting paid for several months of the new expansions use, vs. the loss of revenue for not "selling" them the expansion. Perhaps tie it to a longer subscription plan commitment.