No, not daft at all.
It's all about one thing - profit.
More customer inquiries can lead to more frequent sailings, and also the possibility of more ferries, which in turn leads to more income. But if you recall my comments, and the figures for Dover, you'll see that whilst freight has remained steady, the arse has dropped out of the passenger/car sector, and a reduced number of sailings, coupled with slower running so as to maximise onboard revenue, are all cost-saving measures.
Fuel is a big factor. A ship's fuel consumption is not an arithmetic progression, it's usually a curve that rises as speed (and therefore power requirement) increases to overcome the increasing drag. Because the ships currently have speed in reserve, it allows the operator to adjust crossing times in order to make the most profit from whatever punters they can attract. What the canny operator is doing is striking a balance between fuel consumption, onboard spend, and time taken to get to the destination. They would have estimated, or researched, how long the average punter would be prepared to spend on their ship. If the ferry takes a week to get to Santander, would it be an attractive option? Probably not. But by upping the crossing time from overnight to 30 hours (or whatever Pete says it is), they're hoping the punters will wear it, and it'll save them fuel and boost onboard profit.
Some light at the end of the tunnel - let us not forget that the advent of Brexit has meant the reintroduction of 'duty-free' onboard sales, and we could eventually see the return of the 'booze cruise' where punters would get onboard, get drunk in the bar, sleep it off, get drunk again, and stagger off the ferry in Portsmouth clutching their duty free goods, never once setting foot in Spain. Not my cup of tea, but they were all the rage at one time!