even before this capital of culture thing, Merseyrail usage has been growing remarkably quickly.
capital of culture hasn't actually brought any additional passengers really, the major new shopping complex, Liverpool ONE has, and that is where most passengers head to these days, hence James Street being so busy all day long. It's also busy going towards Chester, because most people get to Wales that way rather than via Crewe.
passenger numbers will only ever increase on operations like Merseyrail, and as the network will expand, i.e. to Wrexham via Bidston, usage will always increase. The thing putting most people of Merseyrail is the lack of capacity, and therefore the rolling stock. The underground stations are also in dire need of refurbishment, but due to restrictions, only Network Rail can do it, and obviously at a high cost.
Merseyrail have the cheapest fares, free car parking, a frequent and reliable service, and is generally one of the best TOCs around. The rolling stock is their only downfall at the moment.
I'm also not convinced the MPV is the problem, it's been here before. If it is, surely that says something about the state of the infrastructure if it can't cope with something other than 507/508 units going over it...