Running water erosion.

A cobblestone/cobbled deepslate block placed under running water should have a small random tick chance to turn into gravel.
Similarly, gravel block placed under running water should have a small random tick chance to turn into sand.
The chance should be very small, to mimic the slow natural process of erosion, e.g. similar to dripstone growth.

Only cobblestone and cobbled deepslate should be subject to erosion, idea being that the gaps in the material let the water to seep in, unlike natural stone/depslate and derived blocks, which have a smooth surface.
Cobblestone/cobbled deepslate stairs, walls or slabs (placed on the upper half of the block below running water) should be either unaffacted or drop as a flint instead.
Only running water should cause erosion not water source blocks.

This change effectively makes sand renewable by means other than the Wandering trader.
The balance is hardly disturbed, however, as the lengthy process is only viable in end-game, when nearby sources of naturally occuring sand had been already exhausted, or for projects requiring huge quantities.
The change also forces a small planning element when building with running water, as gravel and sand are gravity affected, unlike cobblestone/cobbled deepslate.

Bonus: If the running water touches an iron block/iron ore block upstream, the gravel should turn into red sand instead, thanks to the presence of ferrous oxides. The iron is not depleted in this way.
Similarly, having dirt upstream could result in clay.