I don't think this trend, if it is really a trend, is limited to Yamaha. I bought an EV ZXA1 new several years ago. All 4 of the bolts holding the 8" driver were less than halfway in and I could hear the driver rattling around when I moved the speaker. I tightened the bolts, didn't even bother to call EV, and have had no problem since. I bought an EV ZX3 new also and about half of all the bolts used in the entire unit, inside and outside, were completely corroded, I mean seriously ugly. No way any QC or final inspector could have missed that. Two emails later, with no questions asked and no explanation, EV sent me a complete set of new bolts, which I installed. Absolutely no problems since.
I would hypothesize that some companies may indeed be cutting back on the quality control and final inspection stages of production with the intent to shift some of the QC burden onto the consumer and the warranty replacement department, the new manufacturing philosophy being to do everything more quickly, not worry so much about doing it right, let the buyers become the QC inspectors, and, for the more reputable companies, to then replace things if and when the buyers complain. The entire cost to EV of those 2 interactions over a total of $1300 worth of speakers was the cost of 1 bolt set plus 1 way shipping, maybe 1% of the total cost or 2% of the wholesale cost. Probably cheaper to the manufacturer than hiring another inspector and slowing the assembly workers down a bit so they can do a better job.
Does anyone from the industry have any reliable information to refute that hypothesis?