Call me a homer, but my short answer is, "myth". Allow me to state my opinions...
I don't think their quality has diminished, per se. I do believe that certain beers did drop off when they moved to their larger system, 2013 KBS being one of them. There was also a weird batch of Breakfast Stout that hit the taproom after expansion. But in my opinion, each of those beers were fixed the next year. Maybe its a perception thing, since I don't really drink any of the mainstream Founders beer from bottles. I love the **** out of All Day from the can, but I don't buy much else. So my experience with distributed beer, as seen from many outside of West Michigan, is very different. As always, YMMV.
As for pricing, even I can agree that $17 is out of line for Big Lushious, when compared to other Backstage releases. My first thought was similar to others, FBS + Rubaeus blend. Which it isn't, as they had this beer on tap for a few days in July. It's on Untappd as Raspberry Chocolate Stout. I didn't personally have it, but I know others who did and it was a lot sweeter than what you would get when mixing FBS and Rubaeus. It was received well when on tap. Should Founders make CBS again, I say no. The train has left the station, it'll never be the same. I think there are so many other breweries that do big imperial stouts, that it would be such a let down for so many. It was great when it came out, because it was one of the first.
As for the weak taplist over the last month or so, the only thing I have heard, is that Founders went through a lot more beer than they expected over the last few months. ArtPrize, a 2.5 week long city-wide art contest, starts up this coming Wednesday and all of downtown GR is a ******** for the entire duration. Last year, almost 250,000 people visited GR in that 2.5 weeks. Founders is a title sponsor (see Mosaic Promise) and is a venue for many pieces of art in the contest. They are holding back beer so that they don't run out during ArtPrize. Is it a valid reason, I think so. Is it going to get better, not sure. Founders is looking to expand further with an offsite brewing location, with aspirations to get to 600,000 barrels of production.
When compared to other breweries in the beer nerd-dom spotlight right now, they can't compete. They don't make the same styles that seem to draw all of the attention. They don't do big juicy IPAs like HF or TG. They don't make wild ales, berliners and saisons like De Garde, Side Project, Tired Hands or (again) HF. So, have they kept up with the competition, with respect to the "hot style of the moment"? No. But, have they declined in what they do, and their own portfolio? No. I believe they have reached "old has been" status among this crazed beer culture. Do they care, not likely.
They have these three words on their logo, "Brewed for us", and I do believe they really brew by those words. Part of me thinks that out of stubbornness, they won't ever distribute CBS or Black Biscuit or whatever this small beer-crazed community wants, because then they'll be brewing for "you", not "us". They have new-to-craft people walking into the taproom every day. They are expanding distribution to new states every year. There taproom is busy, all the damn time. There's a lineup around the building every Dec 1st when open mug club memberships go on sale. They're doing it right in my opinion.