Look at @tliving's post here about some costs to think about for using Superchargers:
Can you use Superchargers for your daily needs? | Tesla Living
Personally, I'm of the opinion that the S and X will remain "free Superhcarger for life" cars. What I (with no inside knowledge) imagine, is when they want to axe that perk, it will happen alongside retirement of the Model S to be replaced by whatever (Model Y for S-3-X-Y)? That way, they don't go against the established paradigm and the "free" users will naturally disappear as the vehicles age and come off the road. The same will likely happen with the X.
Now, the 3, I can imagine it changing here. Maybe only Signature 3s get it free for life versus paying X per kWh or have Y charges per month. Or maybe they will still get free for life, but have a higher buy-in vs. being part of base pricing (60s were paying $2,500 I believe). This must be tempered though if they release a 3 that has less range than a S. If so, then they may need the free Superchargers to incentivize the mass-market buyers.