because the coin should have a usecase ...and for zcoin its the "payment possibility"
and if there are no coins out ...because all are locked its getting useless..thats why
simple....
also as the masternodes are "taking care" of the transactions, if all coins are locked ...then there arent transactions anymore (very very less) so no need for a masternode as well
Well, it's a bit more than that, it's: (from the white paper)
Zerocoin is a distributed e-cash scheme that provides strong user anonymity and coin security under the assumption that there is a distributed, online, append-only transaction store.
I think you're referring to high velocity and this is actually something you want to prevent:
https://www.coindesk.com/blockchain-token-velocity-problem/As masternodes require quite a high amount of coins, the risk of it all being used for masternodes is very low, Currently we're at just below 60%:
https://masternodes.online/currencies/XZC/ This is similar to DASH (59.3%) and CROWN (59+7%).
We're only at 4.5 mln XZC of the final 21.4 million. Restricting it to a certain amount would mean it is very hard to get a new masternode for a new entrant. Also, the 2500 masternodes would only be about 10% of the final distribution of the coin, so far too low.