The “Last” bitcoin or zcash pragmatically speaking never gets mined because the reward and the units are essentially infinitely divisible. The idea feels absurd today, but in a long timeline suppose a $1.00 per satoshi valuation on the bitcoin network. Because crypto coins are so divisible their theoretical final-day crises are avoided so long as valuation keeps climbing. Add in the future prospects of enhancing the fee mechanics of a vast and valuable network like Bitcoin and there are many ways to assure sustainable network security participation indefinitely.
Zcash intends to use a fee capturing and recycling upgrade called the Zcash Sustainability Fund.
The upgrade to proof of stake also supports Zcash or any other consensus equitable network and its ability to incentivize security participation.