When you go looking for a decline, your looking for future performance no? Peripherals include more than FIP/xFIP. In the second post you reference I mentioned the peripherals I was talking about and xFIP wasn't included. Just put me on ignore if I bug you that much. I'm not going to change the way I post or what I'm interested to please you.
Way too interesting and smart to put on Ignore.
Here's a stab at bigger picture and some history--and I will put your personal preferences aside---as you wish. This will bore some of you--just scroll if you wish.
First baseball statistician that I recall was Allan Roth, who worked for Dodgers--starting in 1950s, I think. Another Branch Rickey innovation. Vin Scully used to refer to Roth's work a lot on Dodgers broadcasts. First heard term "on base percentage" in maybe 1959 from Scully, a stat you couldn't find anywhere until many years later. In the mid-1960s a guy named Earnshaw Cook wrote a stats book (Percentage Baseball) that was mostly wrong about everything but I waded through that, understood maybe 10%, and a few things the guy said didn't seem right but were interesting. Then, came Bill James in late 1970s with his Abstracts, the first few of which were typed and mimeographed and mailed to folks who responded to an ad in the Sporting News, Pete Palmer (Hidden Game of Baseball), Craig Wright (Diamond Appaised) and then a bevy of non-stop sabermetric work, some great, some interesting, and some nonsense.
At some point, folks started wondering about the luck factor in baseball--the McCracken study. But, the roots of that go back to much of the previous work above, seeking to find objective measurements. What to do with luck/random chance, something we all know is inherent in baseball and sometimes breaks our hearts. Last night, the hard DP grounder by Marte was a crucial DP turned by Russell/Castro but could easily have been a two-run single that changed the whole complexion of game. A few feet hit to the left or to the right--all the shutout stats talked about today about Arrieta--- out the window. Maybe Cubs still win--who knows. Luck mostly, I think, on that batted ball. Good work by the defense but that depended on location of the batted ball. Of course, this happens everyday during the season.
xFIP, FIP, xFIP- and a bunch of other modern stats, that's what they do: take out the reality of what happened--the luck/random chance factor--and look at what pitcher "can control" aside from what actually happens to balls put in play. They are not simply predictive. They purport to tell us how well the pitcher performed, not just in the future. You can use that stuff whatever way you want, but it is designed to measure performance. Predictive accuracy is the cherry--look at us and these stats--we can better predict the future. That is fine, but a corollary to what the stat actually purports to measure.
ERA has the sin of actually taking into account that the Arrieta pitch turned into a DP-- rather than a two-RBI ground single that kept the inning going. Yes, ERA has shortcomings but there is a reason smart baseball "insiders" and fans constantly talk about ERA. Few of these folks are knuckleheads. Not sure I can say the same for those other folks who discount ERA and find it unworthy of consideration, not including our esteemed fellow poster--who is not a knucklehead (mostly).
Taking out the luck/random chance element and measuring what is left---a noble effort. Useful. Glad folks do that. We all should look at those stats. But, ERA is important. We care that the Marte grounder turned into a DP and assessing any player or players statistically should too. Put another way, no earned runs for Arrieta yesterday, thankfully. Go Cubs. NOT unworthy of consideration.