I suppose it would make it much easier for them to say we are introducing services to Hull. Wind up the OAO and just incorporate into their franchise. No loss of money for first as it is still them providing the service.
I think the key consideration is, how much does Hull Trains actually make, and how much value do First place on the security of tenure they have on the HT paths. In some ways an OAO operation is a lot more secure in the long term than a franchised operator. Its also worth remembering that HT is 20% owned by Renaisance Trains.