Perhaps I am taking a longer view - but any business should be following some sort of 3|5|7 year planning model ...
You should have a longer retrospective view on what has been happening since 7*2 years ago when IPv6 was standardized.
I do ... and have seen many "This is the year of IPv6" style statements, which is not what I am saying. And again, I am not saying IPv6 was perfect. Are you saying you don't think anything relevant has changed in the last decade? Sorry, but I wholeheartedly disagree - I see that quite a bit has changed WRT IPv6. The protocol itself has been tweaked, the implementations of the protocol have evolved, the deployment guidelines for both and the understanding of all three of those things has been escalating rapidly over the last ~couple-few years ...
So, does anyone NEED IPv6 deployed in their network TODAY ... Perhaps not. But if they haven't started the process (or at the very least evaluated the impacts of doing|not doing so!), and fall into the above category(ies), they are being short-sighted and perhaps even negligent. ((Still IMHO))
Why, do you think, that arguments similar to yours were popular 15 years ago but not now?
(Asnwering what I think you meant to ask ...) Things hadn't progressed on some fronts (and degraded in others) to the point they are at now. As with any other fundamental change, it takes time to reach a threshold (a "tipping point", if you will). Are there still certain failures in how this whole deployment (or some would say, lack thereof) has transpired? Sure. Are there a few things that could have been done different (some would say, better)? Sure. Are any of those deal-breakers? IMHO, nope. In fact, "the perfect is the enemy of the good" - inaction due to certain failings is often worse than the alternative.
What, do you think, have happened to those who followed the arguments 15 years ago?
I believe that is something of a strawman. Naturally, if they were banking solely on an all_IPv6_only_IPv6 world they are most likely no longer around. I am not arguing for anyone to punt IPv4 completely, for quite some time anyway. BUT there are quite a few organizations that have been preparing for, and some actually running native IPv6, for several+ years ... and I firmly believe they will find themselves far more prepared for the time when we reach that "tipping point", and thus in a strategically beneficial position. /TJ PS - I ask again, what is your opinion of all of these organizations that are actively pursuing IPv6 deployments, encouraging environments to do so, etc. ... ?