Vintage and PSA's grading consistency over time

[the 'stache]

PSA started their business in 1998. So, we're coming up on two full decades that they have been grading baseball cards. My question is this: over time, how consistent has PSA remained when grading vintage cards? I ask because it seems that recently graded vintage cards are showing far lower grades than I would expect. Am I just missing things, or have they perhaps become more critical in their evaluations?

Here's a hypothetical. Say we were to take three different 1952 Topps Mickey Mantle baseball cards, and all have been graded a "7" by PSA.

The first was graded shortly after the company began, so in 1998 or 1999.
The second was graded in 2007, or 2008.
The third was recently graded, in 2016.

If I were to compare these cards side by side, how much disparity would I see between the conditional quality of these examples?

There will always be some variation, because it is human beings that do the grading. But, have their written standards changed over time? I wasn't collecting when they started business. In fact, I've only been "back" for the past five years, or so. But it sure seems that vintage cards freshly graded by PSA are seeing tougher grades than before.

What's the likelihood that we'll see PSA (or more accurately, PSA's graders) relax those standards somewhat in the future? And will cards getting 6s and 7s now get cracked in order to get 7s and 8s in a few years? One would think that the key to PSA's reputation would be consistency over time. A PSA 7 in 1998 should be the same as a PSA 7 in 2016. If all PSA 7s are not created the same, what's the use in having a company grade cards at all?