I think that's very much the case. I don't wish to colour the subject with politics, but given the controversy over the denied LO takeover of Southeastern metro routes with the "I agree but we need to keep this out of the hands of a Labour mayor" statement and the fact that we still have the same transport secretary and now also have a labour mayor, I would entirely expect the DfT to try and undermine the case for LO getting any more suburban rail as much as possible. A disaster with the Goblin, whoever's fault it actually is, will certainly go a long way to achieve that.
I understand the point you make but I think things have moved on a little bit. It is Grayling himself who has made two statements to MPs raising the possibility of transferring some West London Line and all GN Inner services to TfL. The Mayoral aspiration and TfL policy hasn't changed since 2000 when Ken Livingstone won the Mayoralty. Even the current Mayor, in recent letters and statements to the London Assembly, has not gone out of his way to slag off Grayling. The tone has been calmer and much more conciliatory and there appears to be an ongoing dialogue between City Hall and DfT. It may still all founder and come to nought but it's a tiny bit more hopeful (for those of us who support devolution).
The GOBLIN problems put a lot of people in a bad light - Bombardier, Network Rail (accountable to Mr Grayling), a number of railway contractors and TfL (accountable to the Mayor). I don't think anyone comes out of it very well and we're still nowhere near having a reliable all electric service on what is my nearest Overground route. Anyone who starts flinging mud around will soon find some of it sticks to them. That's probably why any public rowing between those involved stopped a long while back.
To return to the subject of the 710s a Mayor's Question and Answer released this week said the following. The Question Time was only held last Thursday (19/7) so this is a recent reply.
Barking to Gospel Oak Overground service (2)
Question No: 2018/1713
Caroline Pidgeon
What contingency planning has Transport for London carried out to ensure that the eight Class172 diesel multiple units (172001 - 72008) will not be transferred to West Midlands Railway before eight New Class710 electric multiple units have entered service on the Barking to Gospel Oak Overground service?
The Mayor
A sublease arrangement for Class172 trains has been agreed until December 2018. The Class 710's manufacturer has assured Transport for London the new trains will be available for service by November.
So the pressure remains on Bombardier to deliver and, if they don't, for someone to deal with the loss of the 172s post December. Note that TfL have not said if there is a further contingency plan in place. I know that there are conflicting reports about precisely when the 172s could leave London but I am assuming the Mayor's reply represents the position as of this week.