Safin and Hewitt had their success in times where there were less dominant players yes, but you have to take into account that conditions and diversity of surfaces were totally different at that point. If today's players played on surfaces like that there might have been a few more upsets the last couple of years, whereas Safin and Hewitt themselves would be hampered playing on today's surfaces.
At the end of the day comparing eras is tough. This era makes it harder to win a slam, but far easier to dominate if you CAN win due to the lack of diversity there used to be.
A second slam and #1 for Murray will take him past Safin and Hewitt. Hewitt is probably the most similar past player to Andy I can recall.
I think Andy is an evolved version of Hewitt. Similar strengths, but he is more athletic and has more weapons.
The issue of surfaces is the one thing that prevents a proper comparison of the two eras.