Building a 13" MBP produces ~200kg of CO2[1]. Taking GP's 50 million figure we get 10 billion tons of CO2. Very generously assume each one gets 1000 updates, you end up with 360,000 tons of CO2 total for all of these updates, or about 0.0036% of the cost of building them in the first place.

There are actual benefits to delta updates but the environmental impact isn't one of them.

[1] https://www.apple.com/environment/pdf/products/notebooks/13-...