My understanding is that teams like Williams are held back by the spending cap, because their facilities are out of date, and they aren't allowed the budget to improve it.
If F1 wants rich people to buy into the sport, they need to find a way to improve a team's infrastructure, while still honouring the cap's principles (otherwise Ferrari could continue developing bleeding edge infrastructure year over year). If Williams isn't allowed to catch up, then how is the team worth anything? It would make more sense to invite new teams who can build infrastructure with unlimited budget. (Is that the true fear of allowing new teams?)
It would be weird to have a standard facilities design, but maybe that level of regulation would allow a fair chance for everyone. (The wind tunnels are already regulated, so maybe not that weird?)