Personally I think that is the wrong analogy, or not sufficiently applied. If I buy (say) metallic paint at purchase then clearly that has to transfer with the car as it is, literally, baked-in. However if I buy (say) alloy wheels at purchase then I can choose to remove them, fit standard steel wheels, and transfer the alloys. So I could (say) buy alloys on a 2018 model 3, then steels on a (say) 2021 model 3, and by swapping the wheels each time end up using those alloys for several years.
With alloy wheels this is not so relevant as it really is not worth the effort. But if I am (say) a road warrior then I may value FSD and be an early adopter. However when I sell the car second-hand the acessible purchaser pool in my area may all be retired dodderers (apart from me, the road warrior) who do not value FSD as they simply do not do long trips or even much driving. Knowing that I would take the likely massive depreciation of FSD-on-resale into account when making my original purchase decision and I might go so far as to not buy it.
This is not a hypothetical consideration for me personally. I could easily envisage buying a 3/Y in a while and I would seriously consider stretching to FSD if I knew I could transfer it. However I definitely will not be buying FSD at that time if I thought I could not transfer it.
For Tesla the transferability option can both motivate accelerated sales of FSD, and motivate Tesla-lock-in as Tesla-FSD would not be transferable to other brands. Clearly in this I am making the simplifying assumption that there would be no backwards transferrability, only forwards transferability. I could also consider a fee for the Tesla transfer cost (such as it is) as the transfer from (say) a 2020 model 3 to (say) a 2023 model 3 might enable additional functionality due to the (by 2023) improved HW (as there will be code-forking and version control issues to manage).
I am not alone in this thought process. That is why it was raised as an issue at the Q&A, and is in fact the subject of its own thread:
Why is FSD not transferable to your next Tesla?
I too would be really interested to see any stats for FSD adoption.
regards, dspp/pp