Horrible, glass lower door wtf. How could they go from the P1 to that.
Because the P1 was slightly compromised by its need to satisfy both road and track requirements, therefore it was not as much of an out and out track car, according to McLaren designers.
So the P1 will be replaced by two cars.
The Senna (P15) is road legal yes, (although only building 500 means they do not have to pass many tests needed for more mass produced road cars) that is not what it was primarily designed for.
The Senna, is focused much much more on going round tracks as fast as possible, hence the far more overt and extreme aero, the big weight loss program (over 200kg lighter than the P1 already), and the real concentration on power to weight ratio and down force.
There will be a P15 GTR mid to end 2018 which will be a pure bred track car, (not road legal at all), and once they have lost another up to possibly 80kg from ditching the stuff needed to make it road legal, it will be even faster, than this P15 which is already faster than the P1.
Later in 2018 there will be the road replacement to the P1, which will hark back to the F1 with its center seat layout, currently code-named BP23, already engineers are claiming it will be the fastest ever McLaren, with a power output in excess of 1000bhp, that is a claim that should be easily achievable, so looking forward to a McLaren that will outperform the F1's top speed of 240mph, and the P1's de-limited top speed of 250.
But there will be no track versions of the BP23 an no spider versions.
The Senna P15 will be the track car, and the BP23 will be the road car, so neither will need to be compromised to be able to do the other aspect.