You're right for the most part Andy.
The one scenario where I know the speed goes up on its own is where AP had previously adjusted down to obey a lower speed limit than originally set.
Example: undivided road speed limit is 50mph, AP set to the max speed (in latest firmware = 50mph, in earlier ones 55).
Car comes up to a new speed limit of 40mph, AP slows down accordingly.
A bit later, speed limit goes back up to 50mph: AP adjusts up again.
Yes, I was aware of that.
But as I think we now agree, that is just Tesla imposing a limit and maintaining that limit, which is based on the speed limit of the road, as that speed limit changes.
I think the feature still missing that a lot of people want is the ability to set TACC at some speed relative to the speed limit--say the speed limit or the speed limit plus X, and then have the car maintain that speed as the speed limit changes, on divided highways, of course. So my wife, who commutes on an interstate and is pretty conservative, would set the speed to speed limit plus 5 MPH, and when the limit changes from 65, where it is for most of the trip, down to 55, the car would slow itself from 70 to 60. (I realize the offset was never demo'd, but very early on we did get an offset exactly like that for the speed assist (warning.) The concept would be the same.)
I recognize that the probable reason Tesla has not yet provided this is that the format of speed limit signs is not consistent, and there could be somewhat catastrophic errors. I think they could probably deal with that with some sort of safety mechanism that causes the car to alert instead of actually change the speed if the differential between old speed and new speed is too great. If they want to get really fancy, they could even geo-locate "overrides", so that if there is one sign on someone's regular route that causes trouble, the safety over-ride could be over-ridden, if that makes sense.
I guess what I'm saying is that this should be a problem the smart folks at Tesla can solve. I'm just concerned that now that there is AP2, there may not be that much incentive for Tesla to continue to improve AP1.