"free" is a catchy albeit misleading term in this case. "no per use fee" is more accurate but less catchy
Indeed, and that is why it is very likely they will offer this with Gen III (the $1,000 to $2,500 charged or built into base cost will cover cost to expand the network).
However let's not inadvertently put words in Elon's mouth,
Elon did not announce that all future cars would have free SuperCharging. He said "the general principal of having free long distance travel, we
want to try to keep that consistent for all future vehicles"
So, again, I think it is most likely going to happen, but he did not promise or announce this. Given the scrutiny Tesla receives from its critics, I think this is more than a technicality that he expressed aspiration but not a promise.
Outside of that point I also find this quite exciting. As I've seen several people point out, this will clearly require more SuperChargers (barring some new solution to long distance travel), so I really like the idea that perhaps as there come to be millions of Teslas on the road, the network will become far more dense. Imagine the SuperChargers spaced 50 miles apart... or even 25 miles apart, that would be really nice added value as a customer.
As to people taking advantage of the no per use ~free aspect of the system... I think Tesla could reasonably put some kind of software limitation that does not allow you to use a SuperCharger within X miles of your home more than Y times per year (i.e., "no, this is free long distance travel, not a replacement of home charging if you happen to live very close to one of our SuperChargers").