I wholly support the idea of having one subscription plan that allows you to play both FFXIV and FFXI. However, I think it's very unlikely for SE to introduce it for the same price as the FFXIV subscription. Instead, I think it would be fair to add a value of $2.99 to the new subscription plan/option so players who just want to play either FFXIV and FFXI without the other can do so without paying more, while players who want to play both don't have to pay too much, as the whole point of merging or introducing one subscription is to pay less than the price of two subscriptions, in addition to giving the incentive to try the other game.

Speaking of FFXI, I think if SE wants to pump some new blood into its playerbase and its life then it would be a great start to introduce a new launcher to it, or even better, unify FFXI and FFXIV into one single launcher instead, both suggestions' main goal is to simplify the rough process of FFXI installation.