Any company still in business has always 'done their own thing and it has always worked for them', I'm afraid. Nothing unique to Yamaha for doing this...
But the drop in product cycle time has GOT to have a reason. Personally, from a marketing standpoint, I feel that the most likely explanation is less than expected sales of the T3...
I certainly remember a MUCH greater sense of excitement and 'wow factor', and a lot more purchases, proud user demos, and just general level of excitement on this forum when the T2 was released, but the T3 seemed somewhat subdued by comparison. Perhaps not quite a big enough leap over the previous T to generate the excitement that the T2 had done...?
As little as Yamaha had to do physically to the T3 case, and the remainder of the improvements being primarily software, it seems to make sense for Yamaha to try and up their product cycle to bring out the T4, and maybe recapture the 'wow!' that the T2 had, and somehow the T3 failed to generate. That doesn't mean the T3 was a failed instrument (before Ian's head explodes!), but user response is user response, and I don't think anyone unbiased would argue that the T2 generated a LOT more excitement than the T3.
I simply hope that Yamaha don't drop into a regular two year cycle... I simply don't see the market as able to absorb that. Part of the value you get from any purchase is how many years you get out of it before it is obsolete, and reducing that window by a whole year changes the equation quite significantly. Even if you are skipping generations, you will have gone from six years to four, a 33% reduction in value. Pretty significant, IMO...
Yamaha haven't used this level of cycle for 15 years or more. If everything that Yamaha does is good, it is hard to find an explanation for them changing what they do. Either it's good now, or it was good then, but it can't be BOTH!
I simply feel bad for those that bought a T3 recently, watching the value of their purchase plummet through no fault of their own. I hope that the other manufacturers don't start to follow this trend...
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An arranger is just a tool. What matters is what you build with it..!