So who is going to pay for these extra units that might be needed 2 or 3 times a year? Do you think that is a good use of resources?
It really doesn't help, with this hyperbole of "only used 2 or 3 times a year". If stock is used not every day, what is to be paid for? Any proper understanding of running costs shows that there is a (small) amount that is time-related, and a (much larger) amount that is mileage-related. Of course, accountants will add all together and reduce this to a simplistic (for them) cost per month, or whatever.
An intelligent rolling stock manager (bit of a short supply nowadays it seems) would have a complete understanding of seasonal requirements. They would think it was madness to scrap perfectly serviceable stock on the basis of some theoretical accounting calculation. They would also understand that passengers do not present themselves in even numbers each day for the convenience of the railway, but that they are in a service industry where meeting paying customers' needs comes high up the priority list.
The (old) GWR, no slouches when it came to financial understanding, built "Excursion Stock" from new, finding it worthwhile to do so. There's a very nice, now 90-year old such vehicle at Didcot museum. It was by no means only used for "excursions", but regularly was turned out for race specials, the Ideal Home exhibition in London, main line reliefs at Christmas ... and at Easter.