I don't believe sales will be an overwhelming issue, at least in proportion to the detriment it would cause SE in the hypothetical instance that the game bombs. While of course, it is entirely possible for it to flop as an FF game (in terms of numbers), it's nigh impossible for the game to be a commercial failure.
The biggest reason why FF games sell, is because of, well, the name. Putting the "Final Fantasy" brand name on something guarantees it will sell. If anything, the ultimate outcome of XIII and other such recent disappointments, can easily be called brand abuse: the game was received so severely, yet it ended up as one of the highest-selling FF titles.
While I've seen a cornucopia of premature purchase strategies for FF XV ("I'll buy it once it hits the bargain bin", "not until I see reviews", etc.) that's more of a testament to how polarizing this game is, than anything else. There will be millions of people such as myself, that will buy this game on launch day or within the immediate span of its release, for one reason or another. Whether it's simply out of being a die-hard FF fan (me), an ARPG fanatic (also me), or out of the sheer desire to play a game that will more than likely have taken almost a decade to make, it's going to sell well for a game of its kind.
XV seems completely to be a terminal epoch of the Final Fantasy franchise: I agree, it's all or nothing. Revival or bust. This game will only deliver one of two extremes, in that it will make or break FF.