PCs are open platform and unlike consoles there's no mandate or 'sharia' that they're all the same specs.
Even putting aside the variable hardware and refresh rates, fair play isn't even a thing in online MP on PC, given the plethora of client-side hacks that can be performed on the platform. The Xbox One and PS4 have yet to be hacked, therefore there are no exploits outside of in-game ones.
Input devices have largely varying results on PC, but it isn't true to the same extent on PS4/Xbox One. Of course, there's stuff like the Elite controller on Xbox One, but the advantages they offer are nowhere near as substantial. Even the 3rd-party mice only emulate analog movement, and therefore aren't anywhere near as advantageous. Arcade sticks are a preference and not a meaningful advantage in fighting games. There are many who prefer to play with controllers.
Internet latency and display lag (top-of-the-line TVs are the ones with high input lag due to a higher degree of post-process, not vice versa lol) are the only two variables involved on XBO (at least up until now) and PS4, and even when you combine them, the advantage that a doubled frame rate ceiling can bring is far more substantial. Matchmaking is pretty decent in most of the multiplayer games I play on the PS4, and I can't remember the last time I was put in a multiplayer match against players with high pings. We get good enough pings with Middle Eastern players, and of course there are also local players.
Regardless of all this, the important thing to note here is that just because one or two variables exist, adding another variable like frame rate (a substantial one at that) only makes things worse.
Isnt it at a normal practice to sacrifice resolution over FPS in MP modes even on the next gen vanilla systems ? Uncharted 4, Gears of War 4 etc while all run at 30 fps in single player always target 60 fps in MP preferring a lower resolution.
Yeah, that's a normal practice, and a good one. However, the practice where, say, a game's frame rate is limited to 30fps on PS4 and the ceiling is raised to 60fps on the PS4 Pro is where it gets problematic. A rare example of this in the PS4 ecosystem is Dark Souls 3. The PvP there is an unbalanced mess as a result, because even though the Pro's performance averages at around 40fps, it's still a noteworthy advantage over the PS4.
Alternatively, a rare example of a game adhering to the same fps ceiling on both the PS4 and PS4 Pro and becoming unbalanced is Battlefield 1. The Pro gets a 10-15fps advantage over the PS4 in taxing scenarios owing to the bump given to its CPU clock speed.
Outside of these two games, there's literally no other game that presents a noteworthy performance advantage for the PS4 Pro in a multiplayer scenario.
only a idiot dev would kill their title by making it 30fps & 60fps on different iterations of the same console.
It's not fair to call Playground Studios idiotic. They're among the best devs in the industry these days. (Note: MS considers Windows a part of the Xbox ecosystem, so kind of the same platform lol).
MS doesn't seem to care about frame rate parity in MP anyway. They allow Xbox One/PC crossplay in some of their existing and future first-party titles (e.g. Forza Horizon 3, with PC players being able to run the game at double the frame rate). The Scorpio will simply be another addition to this variable ecosystem.