There is some cost to integrating renewables into the grid, though most experts seem to suggest the first 80% or so isn't that hard, and we're so far away from that in most places that the positive externalities massively outweigh any negative.

If people did use this model to get to 100% then presumably the final 20% or so would be more expensive, since you'd still be paying grid prices for your usage, then selling the same amount of renewable power to try to cancel out that cost. If that renewable power is worth much less, because of integration costs, then you'll be paying much higher costs overall.

(Note, paying much higher costs is a good thing, since it gives a market incentive for people to invest in storage technologies, demand/response, efficiency etc. This is similar to big companies like Microsoft changing an internal carbon tax to redirect investment).