So, FIA just increased the minimum weight by 6 Kg to allow for the Halo.
The result is in the 2018 car comparison thread:
All cars became longer to converge with the already long Mercedes (yes, including Sauber). Clearly, they really needed those 6 Kgs...
We have had weight increases to allow for KERS, to allow for wider cars, to allow for wider tires, to allow for Hans IIRC, to allow for "tall" drivers, to allow for the turbo and the batteries... at the same time, the cars became longer, the aero appendages more complicated, the suspensions more and more sophisticated, all sort of air conducts appeared here and there...
F1 is an aero dominated formula. The best aero normally wins. Hence, any liberty in car design allowed to the teams, will be traded for aero performance (or suspension performance). It would also be traded for power, but the powertrain already has a generous minimum weight.
And then someone complains that they are at the weight limit...
Simply, right now, cut car weight by 50 Kg. It is a figure change in the regulations. A single figure. Done! If that happens, next year all teams would show up with cars about 50 cm shorter, which would be choppier but actually faster cars. Everybody happy.
Why is this so difficult to understand?
For every complex problem there is an answer that is clear, simple, and wrong. TANSTAAFL!