It's not flagship fatigue - it's mid-market distraction.
While I think releasing a new flagship phone every 12 months or less is bound to saturate an already highly competitive market, I believe the reason for the recently-observed FF effect is also due to the fact that the likes of Motorola, ThL, DooGee and Xiaomi are deliberately not aiming for these high-end segments.
There's a new segment that's opened up, which all of the top manufacturers have zero products in. It's not the cheap end of the market and it's certainly not the "mini" versions of the flagship models market, either.
This new segment is led by the likes of the Moto G - one could argue that Motorola really started it - whereby the product is not an all-singing, all-dancing device, it's not a mini-me version and it's certainly not at the other, cheap and nasty, sluggish with a crappy locked-in interface (unless you root it, of course) end either.
This new segment brings clarity to what the smartphone ought to be; yes, it's got features but not any unnecessary features (like a grafted UI on top of the OS), or smack-you-in-the-face speed processors or indeed gigantic screens (although the One+ models go against this mantra) and certainly no gimmicky features, like heart monitors and so on, that barely anyone uses.
This new segment focusses on clarity, speed through simplicity and a peeling back of the OS, to reveal the original beating heart of Android (I'm talking a stock version), like the Moto G and E and to a degree, the X. One+ is doing the same with their Cyanogen image. This new freedom to offer a very good performing phone, at an unbeatable price (between £100 and £200) was unprecedented, until Motorola released the Moto G, in 2013.
Google's current slogan is "Be together but not the same" or something like that. I think Android vendors have had too much freedom or not enough policing from Google and the result has been Apple's runaway success, because they have always presented a consistent look'n'feel to their products. Sure, freedom is good and messing about with the UI is nice but really, do you need all the grafted interfaces, from Samsung, hTC, Huawei, Sony, LG etc? I would argue no, because plenty people bought the Moto G and never did I read anyone moaning that it was 'too stripped down'; if anything, industry pundits LOVED that about it!
My wife was delighted with it, because it had a competitive screen size, it was very cheap at Tesco, on a PAYG deal, it had fantastic performance and there was absolutely zero bloatware on it and Android was sparsely populated (as it should be), so users can make their own choices about apps. Couple that with a simple app that xfered all your stuff from the old phone to the new one and hey presto! A new market segment was invented.
This, in my humble opinion, is where Samsung and Apple are haemorrhaging customers to. The abandonment of the flagship segment is not a net migration to Huawei, Doro or Alcatel and the like, but an embracing of Motorola, One+ and other manufacturers, with a 'natural' product, performance that delights and at a very attractive price-point