Of course, it is tyre company testing, not race team testing.
It does not have to be 21 other cars, it could be 10 other cars (1 car per team). Who says Pirelli has to bear the whole cost of testing? This is what I meant by saying all the involved parties sharing the responsibility of additional resources required. The way it is done now is unprofessional sporting.
According to Hembury (so keep you bs detector honed) when Pirelli first mooted the idea of the test that they eventually carried out in Barcelona with Mercedes, they asked every team if they would be interested in participating on the basis that they, Pirelli, would pay to hire the track and would cover the teams‘ costs. 1 out of 12 teams responded. You think the level of interest would improve if the teams had to pay?
There could be a mandatory system where teams have to pay the costs, or a special levy on teams to create a fund to pay these costs, but as a team owner I would tend towards the view that Pirelli has agreed to supply tyres for a fixed price, and they‘ve agreed to treat everybody fairly, so to the extent that testing is needed in order for them to make suitable, safe tyres, Pirelli ought to have factored the cost of doing that testing in a contractual and fair way into its price.