Here's an interesting point that hasn't been brought up yeat.
This interview with Boullier at the end of 2017.
Q: Eric, you said that it dawned on you even at the Barcelona tests in February that 2017 would be another disastrous year. Can you talk us through the discussions that set in at that very moment?
Eric Boullier: That’s easily told: I went to the management, showed them the data and told them that we cannot accept another year like this. We had a tough first year with Honda, we had a tough second year, and had expected progress good enough to get us back to where we belong – but Barcelona showed that we would go backwards and that was absolutely not an option. I obviously warned them about the consequences of another year of no results, where you keep everybody afloat. We have a new team, which has been reconstructed in the last three years: new people, very good new people – competitive people who used to win – and the danger was we’d lose them. The perception of a team is still very much based on drivers, because they are the faces of the team, but for me the real danger was losing those people. That was the discussion at the very beginning of the season.
So from his answer, I think it's safe to say that he was the one who convinced management to dump Honda. Making a case for Renault would get them Red Bull as their benchmark for how well the car is actually performing, and they've failed to hit anywhere close to it.
If I had to guess, management was starting to regret getting rid of Honda after seeing Honda's progress this year and McLaren's continued struggle. Boullier didn't help his case at all when he
admitted last week that customer teams can't win championships. In the end, McLaren worked hard for 3 years to give Red Bull works status.