People are making it far far too simple and jumping to conclusions while also ignoring what ground effect would do to the sport as it is.
A fundamental part of the sport is ongoing updates throughout the year and teams making better chassis winning. Ground effect is to a certain degree dangerous, to implement it which is relatively easy you can test the limits and where it may detach, but it also somewhat precludes in season development because lack of testing could lead to fairly dangerous situations.
It would also likely mean almost every car with heavily regulated ground effect that doesn't change throughout the year and same on every car needs a similar design of car, same rake, same suspension setup, same almost everything. It works in GP2 precisely because it's a stock car series, it could be a huge problem in F1 precisely because F1 is not supposed to be a series that forces near identical cars. Gp2 also has an entirely different format to consistently mix up the order. GP2 in F1 race weekend format would have drastically less variation in the finishing order.
With a significantly ground effect aero then wing aero would be reduced in effect proportionally making the differences between cars way smaller, not bad but also not at all what makes F1 what it is. Most of the effort currently within the wing is done because the wing lines up with the edge of the tire and they are trying to work as much air outside of the tire as possible. A narrower(as I understood the intention was but might not still be) along with a wider tire means 70% of the intricate crap on the end of the wing becomes worthless.
Diffuser being bigger likely means more reliable downforce in corners, this is what is needed for closer following. The front wing by design hits dirty air when following but the entire length of the car is about shaping the air to get to the diffuser thus the diffuser works a lot better than the front wing in dirty air.
They also have wider tires and likely a lot more mechanical grip as a result, again this will probably help in the corners.
It's not just a case of add loads of reliable downforce, it's a balance. Add X points of downforce in corners and ground effect which don't forget will make the lead car slower again. Doing everything the fans think of together could create a situation where the car ahead will always have so much downforce, the car behind can follow so easily that EVERY single straight will have the car behind overtake too easily. But everyone ignores that possibility entirely.
Higher speeds down straights with more grip and less wheel spin down every straight means..... more air resistance. The faster you go the higher drag gets and again the better the slipstream effect(regardless of DRS or not). The lead car is punching a whole through the air and the car behind is following with far effective force.
Every change has an impact and the goal is to bring cars closer, not make it impossible for a car ahead to maintain a lead at all. The goal is balance, a large portion of fans have spent the past 18 months complaining about lap times, how 'close' WEC/GP2 cars are in speed and how much slower the cars are than 5 years ago. There is no perfect solution to every fans complaint and more importantly most fans don't have a clue what they are talking about.
They should make relatively smaller changes, wider tires is a good step, some smaller aero changes, see how 2017 is. If passing isn't improved, consider another step from there.
AS much as fans think there is, there isn't a silver bullet, fans don't think beyond the big change they think will work, ignoring all the knock on effects of that change. Hitting a specific point where over taking is decent but not insanely easy is incredibly complex.