IMO drivers do make a difference... lets use the best development driver on the grid for example.
How many races have Macca won since LH left? How many races did Merc win before LH joined? obviously that is not even mainly down to LH but to say that those are two random facts would be foolish.
Why did Merc need to do a special analysis to figure out why he flogged NR in Mal by over seconds whilst using less power and tyres despite it being only his first race after major regs changes and NR having had much more testing time and the advantage of knowledge gained fro almost the entire race distance in Aus? if its all down to the clever engineers NR should have won Mal 2014 comfortably.
In 2009 Macca F'ed with the new regs but by the end of the season they had probably the best car - who was doing the bulk of the testing that year?
Despite having built a good car under most years of the last set of regs Macca started from scratch every year and had to rely on the quality of their drivers to bail them out when the risks of starting from scratch instead of developing incrementally (as Redbulld did) did not pay off. Another interesting coincidence is that whilst LH was with them the managed to produce cars that were there or thereabout but once he left they made two dogs in a row for the first time in years - despite no major changes in the reg.
I can remember in 2012 after it had become clear that LH was leaving for Merc a straight line aero test needed to be done urgently on a particular race weekend. But there was a little problem. I was raining and the aero test needed do be done on slicks at a minimum speed of about 100mph to get good data.... so who did Macca send out to risk his neck to get the right data for them, the driver who was leaving them or the one who was staying? I think you can guess as you would probably have made the same choice if you were them.
Now compare that record to probably the worst development driver in F1 currently - at least among the drivers with decent careers:
We do not need to go over the last two diabolical seasons that Macca has endured with him as lead driver.... and the fact that the big wigs at Macca wanted rid from 2014 but Fred would not come and then in 2015 his old chums at Honda saved his bacon after Ron had already decided to cut him loose.
But what about 2012 when he was being lapped by his team mate after going down a wrong set up path .... for months!! or 2009 when he took a "monster of a car" at the start of a season down a development path that by August had him asking on the radio "how did we make this car so bad?"
Or last couple of years at Honda when they produced utter shite for cars ..... coincidentally with JB as lead driver.... but nothing to do with him, right? Only engineers matter.
...... So yes, it is possible that FA has some responsibility for what has happened to Ferrari..... It is a lazy to simply unquestioningly accept the received wisdom that it is only the teams of engineers that are responsible for car performance when there is some evidence to the contrary that needs to be examined.