On what planet are they benefitting? Are Williams allowed to build new infrastructure outside of the cost cap like AM and Mclaren? No.
So they have to compromise between spending money on the race car and spending money on improving the infrastructure under the cost cap. So they will remain towards the lower end of the pack for many many years.
They didnt bring a 3rd chassis because of the cost cap. The same way Mclaren chose to take a hit on straight line speed at Saudi Arabia because of the cost cap. Because teams are limited in the number of rear wings they can produce so are down to low and high downforce designs with adjustable gurneys. Its all about compromise because of the cost cap.