These lower ERA's are here to stay and they have been for a while. Since 2007 pitchers have become dominate. That's simply because steroids and PED's -- even though they have been pretty prevalent lately-- have gotten washed out of the game a lot compared to about 15-years-ago.
I don't think you can compare today's game with then because the mound was a different height until 1969 and the strike zone was a bigger then, too. Plus the way pitchers attacked batters is a lot different. Guys in the 50's,60's,70's would put fear in you at the plate. You tried to take away the inside half on Nolan Ryan you were going to be laying on the ground because he was gonna throw you some chin music. Now-a-days you even throw in just a little bit guys whine about it.
But I do think the lower ERA's are here to stay. Just look at how rare it's been in the last 10 years to see some guy hit 50 homers. Hell, in the 90's it seemed like everyone was belting 50+ every year.