Debugging a problem can be a hassle. To justify that, most shops will sell the rebuild or swap. This is especially true because most vehicle owners wait until the last minute, torturing their drivetrain, prior to having it looked at.
The """it started acting up last year but wasn't all that bad""" consumer gets *#@$&ing annoying after awhile.
Bigger shops have a 'salesman' who will always try to sell you the new transmission for every issue, even if just a bad plug, sensor, injector......yep, many transmission get rebuilt because the vehicle needs a tuneup. Toss in known issues, like band adjustments, failing solenoids, other electronic problems(TPS, VSS, wiring corrosion), and just about ANYONE having any shift/slip issue will end up being sold a new unit.
IMO, if you wait for the problem to become critical, drive while multiple quarts low on fluids, never perform some basic simple maintenance..... you deserve to get fleeced by the crooked repair shop. What comes around goes around! Neglect/abuse your vehicle and some shop will abuse your Mastercharge.