Mclaren's 2018 performance basically shows how many things have been wrong in their organisation for a long time. Honda is to blame for many issues, but Mclaren isn't blameless at all.
If Honda would have come back in 2015 with TR, I'd wager they would have been up to performance speed faster than now. The way of working from Mclaren didn't promote a synergetic cooperation in which both parties thrive. The improvements they were able to make last season stems from a large part from TR's willingness to take penalties, to test out varies things and to accommodate Honda where necessary.
Good news is that Franz Tost confirmed yesterday that TR is willing to perform that role again in 2019 for the benefit of RB. With the complete rear end identical, RB basically has a test mule with the TR in 2019. That is a MASSIVE benefit.
Yes it is a massive benefit, but one that no team should have. This culture of 'B' teams is one that really needs to be clamped down upon otherwise we risk plunging further into a 2-tier formula. All the top teams are guilty but I think RB really does push the limits by officially having a 'B' team who will sacrifice themselves massively for the parent team. I don't think Sauber would take penalties to allow Ferrari some free testing.