I can see the need for platform dispatchers at Central (and possibly some of the other underground stations at peak times) but other than that, Merseyrail is perfectly suitable for DOO. A security presence on trains after 9pm would be welcome, but that's a general observation for any metro network, not just Merseyrail, there are unfortunately a number of unpleasant characters around.
But the RMT's primary objection is not so much about who opens the doors, but their ability to bring the network to a halt. Merseyrail Guards are relatively well paid for the area, they haven't to my understanding struggled to recruit, even for the temporary posts. If they become non-essential to the operation of the railway I can absolutely see a pay freeze or replacement with cheaper OBSs. But Merseyrail do need to control costs, fares against the marginal cost of a motor car are uncompetitive, particularly when multiple people are in the car, which makes fare increases difficult to justify, so operational costs need to come down as Merseytravel and the councils don't have money to spare. Ticket Machines and DOO/DCO are an inevitable part of this.