Island Sanctuary supply dramatically shifting in the background

[TwistedTea]

I don't necessarily see the issue with having the workshop though...

Its the greatest source, yes, but you can still just ignore it and do exchanges. There's also nothing forcing anyone to do the workshop daily & you can also schedule things in advance if you don't care about the supply & demand chart and just want to list things and leave for a week.

Without the workshop, meaning just selling crops/granary mats and pasture leavings to the Exporter, it's around ~2372

We also get 2k a week from Challenge Logs.

500k/4372 = 114 weeks

So there's something called the Lazy Man Presets which we can set for a week.
LMP does not care about supply and demand,uses no crops/leavings/granary mats and makes about ~10k/week from the workshop.

Combining the 10k from workshop and Challenge logs: 12k

500k/12k = ~42 weeks

If players approached this without even minimal workshop optimization that's nearly a year.

Doable?

Yes.

Questionable game design decision by SE devs?

Also, yes.

This is still "base game" content in terms of IS, who knows what future updates to it will bring.

Hopefully, it won't take as many iterations as Diadem/Eureka for SE to get IS right.