I think it's the result of judging yesterday's players by today's standards. By today's standards, a Murray with a US Open title might indeed have a claim to have had a better year than any of those of Sampras or Borg, because:
a. Murray would have two Slam titles, and neither Sampras nor Borg ever won three in a year.
b. Murray would have been in all four Slam finals, and neither Sampras nor Borg ever achieved that.
c. Murray would also have the Olympic title.
However, today's standards aren't the universal, timeless standards by which all players from all times must be judged. Things have changed somewhat since the mid-to-late 1990s, when Sampras was around, and they have changed a lot since the late 1970s, when Borg was in his heyday. In particular:
a. There was no tennis at the Olympics when Borg was around, and the event was really not very important when Sampras was around. Barcelona 1992 got a good field but tons of upsets. Atlanta 1996 and Sydney 2000 had rather mediocre fields to begin with.
b. The Australian Open wasn't very important in Borg's day. It was much more important in Sampras's day, but arguably not as important as it is today.
c. The year-end events were very important during both the Borg and the Sampras era.
d. The Masters 1000 series didn't really exist in Borg's era and wasn't compulsory in Sampras's and so wasn't nearly as important as it is today.
e. The regular ATP events were thus slightly more important than they are today.
A Murray with the US Open title would have five titles from nine finals in 2016. He'd have won Wimbledon, the US Open, the Olympics, Rome, and Queen's, and lost the final of the Australian Open, Roland Garros, Madrid, and Cincinnati. A great year, yes. But here's Sampras's 1994:
won the Australian Open and Wimbledon.
won the ATP Tour World Championships, which is now the tour finals. [for the sake of argument, let's call this equivalent to Murray's Olympic title.]
won Indian Wells, Miami, and Rome. [Three MS 1000 events to Murray's one].
won Sydney, Osaka, Tokyo, and Antwerp. [Four regular tour events to Murray's one].
won titles on grass, clay, outdoor hard, and indoor carpet. [There weren't many indoor hard events back then, anyway Murray hasn't won any indoor events yet in 2016].
had a match record of 77-12 [86.52%, Murray with a US Open title would be, assuming he didn't benefit from any withdrawals, 57-7, or 81.43%].
reached 12 finals overall, losing Queen's and the Grand Slam Cup.
I think that's at least comparable to Murray's 2016, even with the US Open title, unless Murray also does something of note during the fall season.
And here's Borg's 1979:
won Roland Garros and Wimbledon.
won 11 other tour titles, including at least a couple of year-end events and precursors to the MS series. The full list is: Richmond WCT, Pepsi Grand Slam, Rotterdam, Monte Carlo, Las Vegas, Bastad, Canada, Palermo, Tokyo Indoor, WCT Challenge Cup, Masters.
won titles on grass, clay, indoor carpet, and outdoor hard.
reached 14 finals overall.
had a match record of 84-6 [93.33%]. Two of those six losses were against McEnroe (three of Murray's seven losses have been against Djokovic). Went 6-0 against Connors, who was 26 for most of 1979, but turned 27 in early September.
Neil Harman@Neil_Harman57
