I was thinking about this myself, which is why I dont understand why things are unfolding the way they are, irreversibly.
In the short run, keeping Luongo and trading Schneider would be the better of the two options. It might get you the player or two that you need to take this team over the top.
I suppose keeping Schneider is a good decision in the long run since he is just starting his career, but the problem down the road would be that the prospect pool is not in good shape. Once the Sedins and Kesler are past their prime... I hate to say it, but this team could be in big trouble with or without Schnieder.
Long term Schneider is definitely the way to go, but like you said, once the Sedin's and Kesler are past their prime, we have nothing - zilch. Schneider will be 33, Luongo's age, in 9 years. In 9 years:
Sedin's - 40
Burrows - 40
Kesler - 32
Bieksa - 40
Hamhuis - 38
Edler - 33
Higgins - 38
Lapierre - 33
Kassian - 30
Ballard - 38
By then, the majority of our core will be in the last year or 2 in their careers or already retired. Then there's also the likely hood of Kesler, Edler, and Lappy have already hit their pick, so they wouldn't be nearly as effective as they are now. I believe trading Schneider gives us the best chance at winning, but only because we would get way more for him than we would for Luongo.
Of course, keeping both goalies isn't a bad thing neither....