I don't buy this argument at all. This is exactly the type of thing experts write policy papers and perform economic research to answer. If your claim were true, we could never debate policy proposals because they all rely on what people will do in the future based on a policy change.

Medicare-for-All is a hypothetical policy change that has lots of research (data and sources) on how things would play out if implemented [1] [2] [3].

[1] https://www.peri.umass.edu/publication/item/1127-economic-an...

[2] https://www.urban.org/research/publication/sanders-single-pa...

[3] https://www.mercatus.org/publications/government-spending/co...