Boycotting itch.io seems to be missing the point (yes, itch.io could have communicated better, but I would assume that the situation is also less than ideal for them and not conducive to making great choices).
Are we also going to boycott Visa or Mastercard or Paypal? If we ever use those again, how is it different from what itch.io is doing? How about artists who use Visa/Mastercard/Paypal, therefore supporting them, are they also the bad guys? Most of us will keep on using those payment processors because "we don't have a choice", at least in the short term. I don't see how itch.io would magically be in a different position, even if they do have more power than individual players, compared to those companies, they have basically no power.