And you don't think that subscriptions to car software updates are coming? They're coming.... All of the automakers are looking at this and seeing another stream of recurring revenue - which will be sorely needed once they switch to EV power trains which will require fewer spare parts.
As a consumer, I love that Tesla does not make money off service. But as an x-exec that created and ran big hw/sw/service businesses for years, I have always thought having service as a non-profit center would become a longer-term problem for Tesla, putting 100% of their financial dependence and focus on pumping out only new vehicles off the line. To that degree, I agree that Tesla like other EV mfgrs will one day have to seek some sort of post-sales revenue streams to shore up their financials.
As importantly though, my experience says if a mfgr has an underlying financial incentive because of annuity services, it causes even more focus to be maintained on customers after the product is sold to retain and grow the post-sales part of the business. If you do the job right, it improves customer sat and brand loyalty, causing the happy owner to purchase your product again, especially as competitive alternatives creep into the marketplace -- i.e. an appropriately priced, packaged and delivered post-sales annuity service can be a win for both the mfgr and owner. As a consumer and owner of my MS, I don't have personal objections towards something like a subscription, IF I know that is the business model I'm getting into ahead of time, AND the business I sign up for meets (or better yet, exceeds) commitments including the value proposition to me, quality and delivery (including timeliness). All of this are some of the very subjective and ancillary reasons I purchased both an 8-yr ESA and pre-paid service -- to hopefully help hedge my bets that Tesla would still care just a little more about me post sale (and post warranty expiration.)
We'll see how that goes. 
BTW, the door is already there for Tesla to create a subscription for the earliest MS owners TODAY. If one reads terms closely, Tesla does not commit to free lifetime on-board cellular connectivity. I questioned two different Tesla employees about this before ordering last Fall, and was told something to the effect "OTA updates and use of on-board internet is free during warranty, but after 4 years, Tesla may charge for ongoing full-time connectivity." Since I was familiar with that sort of model from Lexus, MBZ and BMW I had owned, I asked what it would cost and if any terms were available I could read in advance. Since no MS had yet hit the 4-year mark, their response was "I'm sure Tesla will make an option available when the time comes".
Lots of things could or may not happen -- it's all supposition on our part. IMHO, if Tesla were not collecting as much info as they are having full-time connectivity today (and even more so in AP vehicles) and probably finding high value in it, Tesla could easily have started charging the earliest owners already. Again IMHO, as the fleet expands especially after some time has passed with successful M3 delivery, the need for even more vehicles providing data will eventually become not as statistically relevant, opening the door wider for Tesla to use onboard Internet connectivity (and perhaps non-safety related post-warranty OTA updates) as a source of ongoing revenue or at least cost recovery by creating a subscription and/or bundling with other service annuities like prepaid service plans.
IMHO it's just a matter of time.
