That's true. If there's lots of flexibility in the energy consumption, then it would be easy to keep adding lots of renewable. And there's lot of potential for demand flexibility.

In reality there's limited flexibility, in part due to laziness and inertia. So adding more solar is giving diminishing returns. Which means adding solar gets harder to do economically as the share of renewable increase.

There need to be better incentives for flexibility in demand (ie push consumer to shift energy usage) and for storage (ie give energy producers bonuses depending on the amount of energy storage they have available).