I wonder if the artist renting the apartment can get their decoration acknowledged as an 'improvement' to the apartment in which case they could require the next tenant (or property owner) to pay the artist some amount when signing the rental contract (in some EU countries at least.)
That's not a thing in the UK. The bulk of tenancies here are ASTs, which make most sense over a period of a few years (rather than decades). Our society is predicated on buying your own home while you're young, so a culture of tenancies where the tenant owns the fixtures and fittings doesn't really make sense here.
Also the artist is dead,
his heirs were far more interested in preserving it than extracting money from it, the art is of the type a landlord is more likely to cover with white gloss paint than market as an enhancement, and an apartment in Birkenhead now apparently destined to be a community creative studio probably had very little value both before and after the outsider art was added.
