The issue with this change is that Statikk Shiv's passive is extremely effective on Yasuo between his dashes, AAs, and Qs while Phantom Dancer's passive has the same function as his E.
For a measly 10% more AS, Yasuo loses out on some decent magic damage, damage that helps him significantly before he starts building botrk or IE. With this change, Yasuo will have to build a less favored item to reach the same previous potential.
That is why people are complaining; it is not as if Yasuo is in a overpowered state currently. 10% AS is not that valuable on him since his Q CDR can be almost capped from the AS of BOTRK, and one will be weaving Qs and AAs in a fight as well.
However, I do agree with you that there is an over-reaction over this nerf.
Mathematically speaking, taken Yasuo's base stats along with the runes needed to get to 100% crit chance with current IE compared to Yasuo's base stats along with runes needed to get to 100% crit chance with new IE only yields less than 1% in difference in DPS.
As well, there is always the choice of building a PD and running some crit damage runes. Although you might have a weaker game after completing the first item, you can reach 100% before completing IE. Ultimately, the power spike is moved a bit later in the game, and although it may be somewhat weaker, it is still there.
Edit: Since this thread is getting somewhat chaotic, I would provide the best solution for Yasuo to this change: Get 10% crit runes. In theory, this creates the least amount of unused stats and only lowers Yasuo's current DPS by 1%.
Mathematically, Yasou already have an excess of AS for Q CDR with BoTRK and SS. He does not need more since unless one can essentially get upwards of 2.1 AS, they will not be able to add in another AA before Q comes off CD. Therefore, PD is not that great of an item on him as he does not need to avoid unit collision since he already has his E, and the extra AS and crit will go to waste.
Running a Ghostblade has the same problem, he does not need the AS and further more, he does not need the cool down reduction. The other source of critical chance, trinity force, also has several pitfalls. Yasuo does not need mana, and this item will make his build extremely expensive.
People, just buy some tier 3 crit runes on Yasuo and ask Rito to increase Q's damage by 1% to compensate.