The disk is likely more cost-effective ( if only they'd continue to manufacture them ). Flash memory also degrades a lot faster than disk, especially if not read periodically.

A well manufactured laser disc can easily last 44 years as -somewhat- proven by the history of the compact disc. A solid state memory chip that can hold over 10 gigabytes of data -at a reasonable cost- cannot say the same. Maybe really good quality chips that nobody would ever pay for alongside a game could last 20 years, but realistically 10 years is a stretch without rewriting the data.

Even Switch 1 catridges are a bit suspect despite most of them being less than 32GB in capacity & thanks to scale being much much cheaper than anything GoG will be able to get their hands on for this.

I mean yes they could get manufacturing costs down to $3 per unit I'm sure, but that kind of defeats the purpose of anyone ever wanting to have a physical copy of a game. I think discs are just a much better long term storage medium.

Flash memory data retention isn't that long unless it's periodically powered up, the higher the levels per cell the worse it is (so, SLC - single level cell, where each cell only encodes 1 bit with 2 values - has far longer memory retention than QLC - quad level cell, where each cell encodes 4 bits with 16 values).

Meanwhile as somebody else already pointed out, stamped CDs and DVDs will last decades (this is not so for recordable ones, though if I remember it correctly even those last longer without content corruption than QLC flash).