I don't hypothetically place him in another era, I just think about what he would have done in his own time if Rafa or Fed or Djokovic had chose soccer instead.
The guy went 3-for-11 in Slam Finals, and here were his opponents in his 8 losses:
2008 Fed @ US
2010 Fed @ W
2011 Nole @ AO
2012 Fed @ W
2013 Nole @ AO
2015 Nole @ AO
2016 Nole @ FO
2018 Nole @ AO
So aside from Fed at the US and Djoker at the French, every time he lost a Final it was literally to the greatest player ever at that particular Slam.
Would he have really lost to anyone else? Let's see. How about SFs? He made 10 of those. Here's who he lost to (*--eventual champion), and not one Slamless player among them.
2009 W Roddick
2010 W Rafa*
2011 FO Rafa*
2011 W Djokovic*
2011 US Djokovic*
2012 AO Djokovic*
2014 FO Rafa*
2015 FO Djokovic
2015 W Federer
2017 FO Wawrinka
And his wins:
2012 US Djokovic
2013 W Djokovic
2016 W Raonic
Really, how much more consistent can you get? He made 21 Slam SFs or better, and aside from Wawa and Roddick (both Slam winners themselves), he faced an ATG either at the peak of their powers or on their best surface (or both) in those matches.
The one time he faced a lesser, Slamless player in the SFs or better, he took care of business.
Murray has 9 QFs, 10 SFs, 8 F, 3 W.
Wawa has 6 QFs, 5 SFs, 1 F, 3 W.
Murray doubles Stan's SF appearances and nearly triples his Finals appearances. No contest IMO.
Hard to say. I guess going through each of those if I had to guess:
2008 U.S Open- Definitely wins without those 3.
2010 Australian- Hard to say. His final performance was dissapointing and mediocre, so could have lost to non Big 3 member. I think Davyenko would have a great shot here, coming off the WTF and losing to Fed in a tough match in the quarters he had chances in but as usual got tight.
2011 Australian- This time his final performance was downright horrible. Hard to say who else wins outside of the Big 3 though, so hard to say. Lets say he wins 1 of 2010 or 2011, but not both.
2012 Wimbledon- For sure.
2013 Australian Open- Wawrinka nearly beat Djokovic in an earlier round and probably would have without a bad line call. Pretty good head to head with Murray too. Since 2015 is also a toss up with Wawrinka I will say he splits again, so wins 1 of 2010 and 2011, and 1 of 2013 or 2015, so 2 of those 4.
2015 Australian Open- read above.
2016 French Open and 2018 Australian Open- My memories of both of these are a bit fuzzy.
Maybe of those 8, like 5 of them which would still be pretty good, and bring him to 8 slams, but in what kind of field now.
Really though this era is pretty much as far as depth, especialy with Del Potro's injuries and Wawrinka not emerging until starting in 2013ish/2014, so in fairness a field without Federer/Nadal/Djokovic would be really weak, and people would be saying Murray and Wawrinka overachieved due to a very weak field now. Not everyone considers it a strong field even with the 3 legends, so just imagine without them.
Edit- Just realized I didnt go through the semis he lost to a Big 3 member yet so the total is probably even a higher guesttimate. Sorry getting late here, am tired. I see your point even more now, but again imagine the field without those 3. Not very strong, even though Murray, Wawrinka, and sometimes Del Potro are quite strong, not much else.