With all the "who was a good investment or not", hindsight is easy. They should have done what RedBull is doing now, let Honda work out their problems in a midfield team and then integrate it into their car... but... lets roll back to 2013.
McLaren was just "freed" from their Daimler ownership but also their funding and support. Plus, Vodafone announced that they were stepping out, even a year sooner than expected. Ron Dennis needed a new source of income plus a partner that could replace Daimler. For 2014 they still had the majority of Vodafone funding and a Mercedes PU but needed a change for 2015 asap.
The deal with Honda was necessary and so was Ron's vision of having two world champions in the car. The best years of the Philip Morris team were with two wold champions in their cars! sponsors would line up and money would be flowing again...
Looking back at the three big sponsor deals that McLaren had, Philip Morris, West and Vodafone, none of them were made my McLaren/Project Four...
In the early eighties Philip Morris basically hired and funded Project Four to run the McLaren team to run their F1 program, West came with Mercedes and Vodafone was also a Daimler deal.
I have no clue what the long term plan is now. In their financial papers they are reporting they are selling heritage cars to fund the F1 program. The investments made during the past years form new and old investors have been used to pay outstanding debts for the car division (they of. course had to loan a large amount of cash to start) and to pay off Ron Dennis (around 200mln pounds).
It almost looks like they are throwing away the next two seasons, not investing on short term small improvements (like expensive drivers) and focus on a good basis and focus on good 2021 season. A fresh start. A new title sponsor, a technical partner to compete against the four works teams....
The top four has the potential to scale up their investments without even blinking an eye. McLaren can't. They need a big plan and fast or scale down and compete against Williams.