[BadHumans]

Many countries introduce taxes on sugary drinks and snacks with varying outcomes[0]. These type of things only go endlessly if you have all or nothing thinking and taxes like this are meant to shape behavior so unless you want people shitting less then no you don't need to tax fiber.

[0] https://en.wikipedia.org/wiki/Sugary_drink_tax#Impact

[spacephysics]

My concern is the behavior shaping is not a simple, single variable equation. Many times these policies have knock on effects that produce a worse outcome for those struggling.

[BadHumans]

We have data on this in the form of cigarette taxes that show good outcomes in preventing people from starting smoking and bad outcomes for existing smokers since they don't quit due to increased prices and just keep smoking while paying more so that this could go either way. So I understand your concern. I just take issue with the reasoning "this can go on endlessly!"