Er... a game that came out in '98 (and then 2000, counting VIII on PC) is hardly evidence for PC support. Those games (the originals, obviously, not the Steam releases) are now defunct (um...and over 10 years old) and SE, until the past couple years, has given almost no support at all to the PC market for their top-tier series. If IX, X and XII had PC release upon release, as well as XIII, you'd have an argument...but...really? Games over ten years old with ports that are elementary isn't an argument...it's evidence that they tried the PC market at a horrible time and, predictably, those releases were abysmal both in sales and reception among the few fans that played them back then.
This, however, is a test; I think that's pretty clear. If this trilogy does well, I'd say it's almost a certainty we a XV release, even if it's not at the same time as consoles. But a release at all would be quite telling, and it all depends on if these flop or not. Not the greatest games to use, but it's what they (and we) have. I have no idea why VII and VIII PC releases (again, the originals) have any bearing on this one... It's an entirely different era altogether, they're no longer relevant in any way. Especially considering that the Steam versions have done relatively well. Hell, even IV is doing alright and that's not even the newest version...they should've released the "Complete IV" edition with The After Years.