You are confusing paying for AP and paying for FSD.
I am not confusing, I am merely citing history and Tesla's messaging: "AP3" computer swap has been told to apply to Model S/X/3 cars with FSD option bought, if they need it. (EAP-only cars are said to not get this because EAP aka Drive on Nav does not need it.) Nobody has even mentioned retrofitting any other hardware for this. I am intimately aware of this history, having followed it and bought cars related to it through 2014-2017.
With drive on nav, EAP is feature-complete for AP2/2.5 cars (though presumably will continue to get incrementally better at doing those features). No need to upgrade anything. You got what you paid for.
Sure, I agree EAP-only cars are not (as announced) being offered retrofits and does not seem to need them.
AP1 cars don't get any upgrades because FSD was never offered for those cars.
It was an example of past (lack of) Autopilot hardware retrofits. Pre-AP was not offered AP1 retrofits, AP1 was not offered AP2 retrofits, AP2 has not been offered AP2.5 retrofits etc. AP1 cars were promised many features that never appeared too, unfortunately, such as "meeting you on the curb", stop-sign recognition, automatic off-ramp based on navigation ("8.1 in December 2016"), real blind-spot detection etc.
But that wasn't my point. My point was, Tesla doesn't have a history of upgrading AP hardware in general. They have made a specific exception: the AP2/2.5 computer for FSD (even that has included the caveat: if necessary). That is an exception to an otherwise simple rule - otherwise the story has been that AP hardware is too prevalent throughout the car for retrofits and that
the 2016 AP2 already has everything needed for Level 5 autonomy (except perhaps CPU/GPU power which would be swapped if needed, which was the only stated caveat at the).
Assuming they might suddenly start retrofitting more than the computer seem IMO a very risky speculation or interpretation of history. I believe AP2 will forever remain AP2, except limited upgrades (e.g. new "AP3" computer) for FSD-enabled cars. I believe AP2.5 will forever remain AP2.5, except limited upgrades (e.g. new "AP3" computer) for FSD-enabled cars. AP3 might remain AP3 forever, without anything from AP4, but who knows so far ahead...
The HUD retrofit conversation for Model 3 is of course separate from this. It is at least plausible Model 3 might include some preparation for such a HUD retrofit (IMO risky to believe so, but at least not unthinkable). But I doubt Model 3 includes any preparation for an "AP3" retrofit, though, beyond swapping the AP computer as the need may be.
FSD is a different set of features. If the car needs a new computer and new sensors, and you already paid for FSD on an AP2/2.5 car, you get all that upgraded free hardware.
This is the crux of our difference in opinion. I just don't believe the latter - the part I underlined - to be likely. New computer yes, new sensors - especially the addition of more sensors - unlikely. I am happy to agree to disagree with you, but just highlighting this difference in our beliefs.
Nowhere has Tesla ever suggested retrofitting anything beyond swapping the AP computer for AP2/2.5 customers who opted for FSD. This also makes sense, especially AP2 processing power is insufficient, Tesla is making their own chip for "AP3", and swapping the computer is a relatively low-effort task (behind the glove compartment in Model S/X, behind the screen/dash in Model 3).
Adding and wiring new sensors or redundancy components throughout the car? Wiring cabling to bumpers and pillars, swapping sensors, possibly cutting new holes and attaching installation points (or swapping large pieces of exterior)...? I find that very unlikely personally. The only precedent we have are the early (paid) Model S parking sensor retrofits and those were on a vastly smaller scale both in pervasiveness and most importantly simply car volume...
AP 2.5 already has new sensors - color cameras (at least in markers) - whereas AP2 does not. AP2.5 also has significant new wiring and fusing related to redundancy. I really don't see Tesla retrofitting those to AP2 and there has been no sign or talk of that. First AP 2.5 only features have also appeared (dashcam) and, again, no talk of offering retrofits to AP2 cars. Sure, FSD has not shipped yet, but again, nowhere has Tesla mentioned retrofitting anything but a new computer...
Perhaps retrofits beyond swapping the AP computer could happen in some very limited fashion, say if steering redundancy required swapping also some electronics unit or limited cabling, or installing the e-fuse or swapping some particular camera if it was really problematic, something like that together with the new computer... but wiring and replacing new sensors around the car to match future "AP3" or beyond? No, that seems really doubtful to me. Think of the enormity of such a task - and the fact that not once has Tesla mentioned offering such a thing.
My belief is Tesla will simply put more limitations on FSD in older cars in software, than upgrade the entire suite to match future improvements in hardware. I fully expect this for my own AP2 FSD Model X, it has already started with the dashcam (though not really AP, and this particular thing might change if "AP3" computer helps with this issue eventually, just noting a point).
Personally I wouldn't trust anything beyond swapping of the AP computer as happening after the fact. There are many examples of Tesla simply moving on to the latest and greatest, and increasingly fewer recent examples of Tesla retrofitting old cars to match.