Back in the days when the discussion was about reducing mid-season test days, the argument was made that big teams would just benefit more from reduced track days by having the availability to compensate with more wind tunnel or simulation work. The funny thing is we went from that into just preseason testing and now into minimal preseason testing. I don't see any point reducing it further really.
Whatever kit they have in the factory, it can never replicate on track conditions - tracks rubbering in, asphalt coarseness, temperature changes, wind conditions, interactions with other cars - eg look at Red Bill's engine cover yesterday.