Hard to compare.
If you look at things in a vacuum then it's obvious todays players are vastly superior in terms of athleticism, there's no arguing that. However to really look at things properly you have to account facilities available, the rules of the time and the players they would be playing with and against and think 'what would x have done in y's time period'. Would Souness in him prime if plucked from the 80s be good in todays game? Not a f**king hope in hell, he'd be left huffing and puffing and without doubt be sent off. However if he was born in 1990 with the same 'natural talent' if you want to call it that, raised through the system with the facilities and played and developed into the game we play today, he'd undoubtedly be top quality (if any of that makes sense ) One of footballs most fascinating questions in my eyes.
I get what you mean, 100%, as I've had the same thoughts myself.
I'd just make a couple of points though; never underestimate just how fit those lads were. The game was played at a breakneck pace back then as well, but we stood out (and won lots) because not only were we fitter, bigger, stronger, faster than most teams, we also played very fast football.
My second point is that yes, Souness would have to modify his game, but I'd argue that he did that when he went to Sampdoria, and showed he was capable of playing different styles, like he played with Scotland back when they had some great players.
I know it's different times, I just think the levels are not as far apart as you think, and that the best players from any era would adapt quickly, especially in their prime.
It is one of footballs most fascinating questions, but as above, special is special in any era, in my view.