For those who may not know, when bags are gate checked you are not charged. At least on Southwest, Delta and American you aren't. I haven't flown Jet Blue.
Note I never said they don't measure, I said that in my experience there hasn't been a change in carry on size for domestic flights. I've seen quite a few measured. The only time I've seen them not fit in the measuring box was when the zipper was open so they were expanded.
You've shared your experience, I shared mine. That's what OP asked for