When a national institution starts to sound like Spın̈al Tap, you know it’s in trouble. Recently, Tate channelled the mythic rock band’s claim that its audience was not shrinking, just “becoming more selective”. In response to a decline in visitor numbers and a cash crisis leading to redundancies, the museum group emphasised “record numbers of young visitors” to Tate Modern (who cares about all those uncool visitors above the age of 35?). Yet in the summer Tate director Maria Balshaw blamed the museum group’s problems on a dearth of 16-24-year-old visitors from continental Europe. So they appeal to youth, but the wrong youth?

This week, Tate Modern will open a blockbuster show that might attract paying adults. But Theatre Picasso draws almost entirely on its own collection of the modernist giant’s works, which should be on permanent view in its free displays anyway, though for some time they have not. It’s a far cry from past exhibitions of Picasso and Matisse, Gauguin, Rauschenberg, and in 2022 Cézanne, boasting superb loans from museums all over the world. Why has Tate seemed to become so … small?

Meanwhile, its rival the National Gallery is expanding. It announced this week that it has accumulated the resources to build an entire new wing, and will drop its policy of only collecting art from before 1900, a reversal of an agreement that only ever helped Tate. The National is openly poaching Tate’s territory after its Van Gogh exhibition established it as a world class gallery for modern art.

This comes hot on the heels of a ravishing rehang this year from Cimabue to – yes – Picasso. It’s going modern, and it even has a new Locatelli bar that seems to have floated in from Rome’s Termini station.

The National Gallery’s and Tate’s approaches to modern art are very different. Tate Modern has always foregrounded the contemporary, refusing to emphasise early 20th century ‘masters’. But the National Gallery can show how art was revolutionised between 1870 and 1920 against the deep history of art it houses. It is perfectly placed to tell the tale Tate Modern doesn’t, of how Cézanne inspired Picasso and Braque, how cubism sparked futurism and suprematism.

The National Gallery announced this week that it has accumulated the resources to build an entire new wing and will drop its policy of only collecting art from before 1900. Photograph: Guy Bell/REX/Shutterstock

But if it’s to collect more modernism, it must stay serious. No purpose would be served by randomly showing a Jeff Koons or two. We don’t need a fifth Tate on Trafalgar Square.

Meanwhile, Tate Liverpool is closed for rebuilding and when I recently visited its temporary home, it felt like a deserted husk complete with grumpy caretaker. The only Tate where I can honestly say I had a good time lately was St Ives, and that was mostly for the views of the beach.

Tate and the National both suffered in the Covid years but it’s the older institution that has come out fighting.

Tate Britain currently stands not for a broad-based idea of British art, but one uninterested in most pre-1800 artists (despite owning the best collection of historic British art in the world), rejecting the 1990s ‘Young British Artist’ generation and avoiding big British art stars, past and present.

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Jenny Saville, for example, has just had a fabulous retrospective at the National Portrait Gallery – another museum outflanking Tate.

Tate seems to be a classic case of an institution that hit on a winning formula some years ago and is trapped by the rules it set itself, stuck in routines that are no longer new. Turbine Hall commissions are still timed to the Frieze Art Fair – even though Frieze is nothing like the event it once was. And it still insists on a definition of the modern that mostly means video, film, performance, installation despite the fact that art now is more eclectic, shifting easily from such media to painting, drawing and sculpture. Tracey Emin’s Tate Modern retrospective next year is a good sign, though, giving star treatment to an artist who has led the way back to painting.

Tate Britain currently stands not for a broad-based idea of British art, but one uninterested in most pre-1800 artists Photograph: Madeleine Buddo/Photo © Tate (Madeleine Buddo)

Perhaps this is the real reason the National Gallery has the wind in its sails: we are not so sure what is avant garde or conservative any more, and the new, these days, is as likely to happen on canvas as on screen. There may be some catch-up in the shortlisting of painter Mohammed Sami for this year’s Turner Prize, run by Tate Britain though staged this year in Bradford. Yet Tate needs more than a couple of moves in the right direction. It needs to break out of its rigid brand.

The overwrought hang at Tate Britain, which has been widely and rightly criticised for its didactic wall texts and glib historical grandstanding, will have to be ditched at some point and replaced with rooms celebrating rather than judging 500 years of national artistic achievement.

As for Tate Modern, it should tell the story of how modernism was born and evolved from fauvism to abstract expressionism with some sense of history, some reverence for greatness. Or if not, now’s the time to consider giving its Picassos and Rothkos to the National Gallery.