One thing people seem to not be talking about is funding. At present I would estimate that McLaren is probably the 5th most well funded F1 team with Renault ahead of them and Mercedes, Ferrari and Red Bull a long way in front of Renault again. It should be no surprise then that McLaren are 5th currently. They simply don't have the budget of the other teams currently, not since the days of Vodafone and Mercedes works teams status.
That is where Zak B comes in trying to bring sponsorship money which so far seems to be working. It is my belief that McLarens only hope for the future is to do what Ferrari does by expanding its road car business and using some of that money to support its Formula 1 team. This can be done incrementally over then next 10 years. That $153 million dollar investment is a good start.
What I believe will be cruicial to do before the 2021 cost cap is to upgrade facilities, get better simulators or what have you. Otherwise obtaining this equipment during that period will probably eat into a teams annual restricted budget. When that is established more of the budget can be spent on personnel year on year.
People love to bash EB but the problem is he can only work with the budget that he is given. There is a good possibility that he simply can't hire the number and scope of engineers that he wants to.
Edit: my atrocious grammar