They should just be like "Thanks for supporting Tesla Model S for the last 10 years! Surprise your car is now unlocked and you have [fog lights, accent lights, tinted mirrors, naivation, 70 kWh pack, etc]!"
Specific to your proposal: many are recent owners, who bought a used car. Should you then split the decision by "how long" they have been supporting Tesla?
As other mentioned, I'm happy with what we paid for. Of course I would be happy to get free things, but I'm not expecting this.
And in fact, we got a lot of "free" stuff over the years.
But with that said, tell me which car company has done it (upgraded someones old car for free just to be nice to a loyal customer)...
I recall that initially, the data connectivity was free for 4 years or something like that.
When that time came, nothing happened, and eventually, Tesla decided that all cars before a certain date got it for life, like ours.
Initially, the nav was based on a third party app, navigon I think, and was at some point completely rebuilt for what we have now, which by my book is so much beter (visual, features, speed, estimates accuracy).
Same for Voice, which was initially also a very basic one, with specific commands only.
Maybe a bit late, but we now have Text integration. It was not even there for years, but was added as an OTA.