I still don't quite understand why it makes sense for McLaren to join FE but for Mercedes to leave.
Is it the cost cap? It's obviously an attractive situation for McLaren, but do Mercedes feel that if they can't outspend their rivals they don't think they'll challenge for wins? 
The cost caps for the first two seasons of Gen3 have been set at €13m per season for teams and €25m for manufacturers across a two year period. So right off the bat I suppose McLaren is looking at €13m per season as a customer powertrain team here whilst Mercedes (had they stayed) would be looking at €25.5m a season to run both the race team and pay for the powertrain development.
But then it's also the case that Mercedes took the decision to leave in August 2021, months before the cost caps were finally agreed and announced that December. At the time they were estimated to be one of, if not the biggest, spender in the paddock, where annual budgets had been rumoured to rise as high as €55-65m.
The 2020-21 FE Season was - frankly - a bit of a mess. Qualifying and by extension many of the races descended into a complete lottery, sporting penalties were being handed all over the place for the most minor of infractions and energy reduction rules were both confusing and caused significant controversy at Valencia. It's absolutely not the season you want to have if you're trying to convince a big spender who's executive board is turning a bit skeptical to stay.
This season we're seeing vast improvements because of FE's efforts to address those issues. New qualifying format, penalties are a near non-issue and a new extra-time rule see to that. In more of a longer term sense, the cost caps address concerns over the growth in the levels of manufacturer spending relative to the growth in realised returns. But none of that was a done-deal when Mercedes executives met in August of 2021.
McLaren's viewpoint is probably quite different. They're seeing how FE has responded to the 2020-21 crisis. They know what the total spending bill is going to be and they know they can strike a deal to buy competitive powertrains. The race-team operation they're buying are proven winners and reigning title champions, so the chances of success are high. Tie that in with the McLaren social media and marketing presence and you might be onto something good.
Edited by Ben1445, 15 May 2022 - 11:26.