Everyone saying 'they should go straight to contactless' has rather missed the point. They are clearly sticking with smartcards, but changing the name.
Since contactless cards can't be written to, even if Merseytravel spent millions of pounds on developing a PAYG system, every bus company that offers services in Merseyside would have to sign a contract with Merseytravel to accept this new method of payment. No doubt most buses would require new ticket machines to be compatible with it, and each individual bus company would see a big shift away from their current operator specific tickets towards contactless PAYG, losing them money. The bus companies are under no obligation whatsoever to enter into such a relationship with Merseytravel, and almost certainly refuse to do so unless all of their costs were met, and they were compensated for all lost revenue in the future from taxpayers money.
In short, unless franchise contracts for bus travel which both specify this kind of PAYG and include the necessary millions of pounds of public money to implement it are to be effected throughout Merseyside this is a bit of a pipe dream - and fairly irrelevant to this thread.
Each train operator would also likely require compensation through their franchise agreement, and there would need to be arrangements put in place for Mersey Ferries - these may be expensive but are probably rather more doable than negotiating with the bus companies. It has also been made clear by the Mayor that this is his aspiration. But it has been an aspiration in many places, for many years, with little or no change.