Viacoin Block Reward Halving happened !
Reward per block: 0.3125 $VIA
So when will we hit 23million?
And what happens next? Hashrate down to almost zero?
you might as well have asked "what will happen when bitcoin hits 21million"
you should read up on how real crypto works,
where do you think transaction fees go, etc
In short, once block reward stops with cryptocoins, the network is hopefully so busy by that time that the transaction fees are high enough to keep mining interesting.