You need to see Mclaren's reaction from their perspective as well. I'm pretty sure that, while probably not written in the contract, the initial promises were that Honda will use 2015 to learn, 2016 they'll begin to be competitive and 2017 they'll battle for wins. That was the plan that everyone signed up for and we were told as much in the initial joint press conferences. Judging from everything we've seen, Mclaren delivered their end of the bargain, or at least to a larger extent than Honda. They had a good car, a good development team ready to take on 2017. Now, from their perspective, looking at Honda, there has been 0 relative progress in these 3 years. They started 2015 a long way back from the front and that's where they are now.
Mclaren has access to way more data than us. They probably know what's expected for spec 4 and at least have a good idea of what the Renault PU can do. In the long run, why would you assume that Honda can build a better PU than Renault, even though Renault upped its investment in F1 significantly? What would be the objective reasoning for thinking that? The objective facts right now are that the Renault PU kept a consistent advantage over the past 3 years and their investment is larger than Honda's.