I calculated a long time ago by looking at typical high value transaction volumes like real estate and automobiles that you only need 50 TPS to have a viable digital currency with market penetration in the upper middle class globally.  The idea that you have to scale to huge TPS is false.  50 TPS is all you need to get the job done for the lower limit.  If you ignore other things like rough consensus attacks, Bitcoin could exist forever fully functional at 8 MB blocks w/ Schnorr sigs under current population levels.  Or, if population does a Fibonacci retracement back to pre-industrial revolution sustainable levels of 500,000,000 then I guess you'd only need 3.5 TPS.