I believe they focused a ton of time and resources getting that part right. As for car development, they did enough in 2013 to prove they could build a chassis then left off development, focusing on 2014 while other teams were still out chasing Red Bull and the end of the old regs. While everybody had issues at the start of 2014, Mercedes had a ton more right than anyone, and kept some gap to the rest of the field ever since.

That said, it hasn't exactly been a cakewalk for Lewis and Toto. 2016 was a disaster for Lewis, reliability-wise and his standing starts. 2017's car was very finicky to set up, and Ferrari, 'stroker' engine or not, pushed Mercedes through mid-2019, and at times had a faster chassis and/or engine that Mercedes at a given race. Even being underpowered Red Bull became a problem, but not having a driver to match up with Max after Ricciardo left tamped down any real chance at either constructors or driver championships.

Still, great chassis, great engine, great driver, great team, and a sh*t ton of money = 7 for 7 with the latest regs.