Let's look at each position. 2B and SS have been raising. SS now hit 20 points higher. 2B now hit 10 points higher. Instead of back in the day, when shortstops hit a measly .230, they now hit a whopping .250. Yea, the emphasis has clearly shifted, they're powerhouse hitters now. Second basemen hitting .250....DEFENSE FIRST! Now, we don't care so much about defense, anyone can play there, right...they can hit .260 now. Yea, now we're offense first.
1B - In the early 70s, they hit .290, by the late 70s, they hit .280. In the 80s they (gasp) dropped into the .270s, and now they're a little over .280. So, when you say "since the 70s, do you mean ".290 70s" or ".280 70s"? Let's split the difference and say .285. So, nothing has changed there.
3B - As stated, little has changed. But they were "in the middle" anyway.
LF - Same kind of issue we had with 1B earlier. Which part of the 70s did you mean? That said, in 2011 it had it's lowest BA. I'd like to see this spread out a few more years to see if that's continuing. Let's say it remains constant from 2011 - So we're talking .280 in the "70s" to the low .270s now. Damn.
CF - Has actually become more of a defensive position than it once was. So while 2B and SS are allowing for a little more offense, more teams are seeing more value out of good defensive CFers, apparently. We got a HUGE drop of from .275 to .265 or so.
RF - Has remained relatively constant.
So when we look at the positions, when you say "moved to the middle" you probably mean more condensed, and yes, because SS and 2B positions went up a whole .010-.020 points or so. Aside from that, not a ton has changed. If anything, emphasis on defense based on position has probably changed based on what position you play. Defense in CF has probably become more valuable over time, while at 2B and SS, it's gotten less valuable.
That said, it's clear as day - there are certainly positions where offense takes a hit because the position is harder to play. 3B is relatively harder to play than COF. Beltre doing as well as he has offensively adds value, because 3B isn't a position like 1B or COF like many people think when it comes to offensive productivity.