These new cars are way different than last years. Larger and much heavier wheels (13" to 18" adding about 50 lbs and tons more rotational mass), front ends that make for cleaner air so those behind can follow much, much closer without losing grip and wearing out front tires, under body aero since it's all downforce now with a much smaller rear wing, no more bargeboards to redirect air around the middle of the car, 10% (instead of 5.75% bio fuel), and better crashworthyness.
In simulations, last years cars are still faster.
Throw in the fact that the winning team gets the least amount of wind tunnel testing and you've got a whole new ball game. Just 10% more wind tunnel time can garner reams of R&D to work on. Mercedes won't get any more additional time until mid season and just what they can figure out in between races.
It's a new era and Mercedes will figure it out and then we'll hear Verstappen whine like a little biatch when Hamilton and Russell start being competitive again. I feel sorry for Leclerc and Perez who're going to have to deal with Verstappen in the meantime.
All the best,
Nonoise