Is the free lifetime supercharger worth it?
Not really.
I have a 2015 with FUSC. I supercharged about 1 time last 12 months (and that was just to test it). Why? - partly because I can only get 50 kw. Now I have a 70 which used to peak at 104. I have supercharged very little in the cars lifetime. Never to anything over 80%. Probably 5 times a year.
But Tesla has throttled many (if not all) older cars with the speed of supercharging - to varying degrees.
At 50 kw, I take another car on trips outside its range. It is true that the other car is a Model 3 - peaks at 140 kw - and is more efficient so the miles gained is even faster than the kw difference. Also, the Model 3 has 310 mile range and the S is down to 216 (although I just went down to 18 from 216 so a full charge might get me higher).
We take the 3 even though we pay for supercharging. Typically it is like a $1 or $2 as we just need a bump. We rarely go over 400 miles. The 3 has been our roadtrip car (over 150 miles) for 18 months and 18k miles and I suspect our total supercharging cost is under $30.
Just know what you are getting with an older Tesla. Fantastic car otherwise - but I might spring for newer if you could.