Remember those heady days of 2010, before the cuts kicked in, when we spent our days coming up with arguments for funding the arts? All the cool kids were doing it: making up Churchill quotations and shouting about Danny Boyle's youth. Back then, you could call Jeremy Hunt a vagina on Radio 4 as long as you spluttered a bit afterwards and everyone fancied Sam West because he stood up in Hyde Park and demanded ballet for everybody. RIGHT ON, WESTY, RIGHT ON.

One argument in particular caught on: the economic case. "For every pound spent on the arts," we'd all parrot smugly, "the government gets, like, a gazillion back in taxes and stuff." The real ratio varied: some said one to two, others one to four. It didn't really matter. Look at War Horse, we screamed. Nobody thought that would work and, shazaam, £11 million profits in four years – a lot more now, another four years on. It's not subsidy. It's investment. The argument was so successful that Maria Miller, Hunt's successor at the DCMS picked it up as policy.

I thought back to that argument on Friday morning, when the Daily Mail revealed that the National Theatre is in advanced talks with Disney Theatricals about creating a stage musical of Pinocchio. According to Baz Bamigboye, the plan is for Enda Walsh and John Tiffany, the team behind Once and The Twits, to adapt the Disney classic in time for Christmas 2016 complete with its famous songs, "When You Wish Upon a Star", "Hi-Diddle-Dee-Dee" and so on.

Er… What on earth is Rufus Norris thinking? The National Theatre is a publically funded organisation. It exists to create the best possible art and receives more than £17 million of Arts Council funding a year to do so. In what possible world is Disney's Pinocchio a legitimate use of that money?

Subsidy exists to enable things that would not otherwise happen, art deemed too risky or too outright unprofitable for the free market. Pinnochio is neither of those things – even if Walsh and Tiffany go to town on it, as they likely will.

True, the development costs might be enormous and, since there are no guarantees in theatre, it could prove a very costly failure. That would make it a risk in the same way that Spider Man: Turn Off The Dark was a risk. But it's not like Disney haven't got the cash. The Lion King has taken over $1 billion on Broadway alone; more than $6 billion worldwide. Even Aladdin's taken £200 million in the last 18 months. Disney can afford to shoulder the risk of a new musical alone.

What about the National - why shouldn't it go for a slice of those profits? Its website boasts that operations now cost £95 million a year – the result of a phenomenally successful decade that has allowed the organisation to expand dramatically. In order to maintain that though, it has to uphold the same sort of turnover. In other words, having had a string of box office hits, it needs a whole load more.

Go back to the economic case: it celebrates the fact that great art can reap financial benefits – big financial benefits in some cases. War Horse was programmed because it might make great art, not because it might make great money. That it did so was a (big) bonus. However, the moment you aim at that outcome directly, the moment you programme for profit rather than to create the best possible art, you're betraying the spirit of subsidy. At that point, funding becomes an investment outright and the National's acting like a commercial producer.

The difference is that it would reinvest those profits, rather than pay out to shareholders. Pinocchio might make a lot of other things – and a lot more great art – possible. (Just as housing associations are selling £1 million high-spec flats to build three affordable ones with the proceeds.) That argument ignores two things: one, that commercially-driven art takes stages away from other work, just as high-spec houses affect affordable living; and two, that growth is a self-defeating aim that will, in time, lead to a demand for even more growth.

This is where the economic case ends up: with arts organisations judged by their bottom line and not by their artistic heights. The National cannot become a puppet for that.