I believe future of tourism and travelling in general is in public transport.
Therefore, the cost of roads for private cars is not something which I would consider as important in this context. There are trains and buses from Germany to Croatia, which one can use if can't pay for motorways.
30 or 25 kuna for one way for catamaran or ferry is price which is unsustainable in long run and I am looking foward for introduction of real, cost based fares for visitors and tourists.
Tourist were visiting islands long before these prices where introduced and will continue to visit I have no doubt about this.