One of the reasons why the side wall is now higher than originally planned for the 18'' wheels was because the feedback from F1 teams was that the low side wall was going to have a too much of a negative effect on the suspension performance. Subsequently the side wall height was increased and together with the 18'' wheels we ended up with the very large 720mm diameter we have not
This negative effect on suspenson performance will propably relate to the following two consideration.
1 With the tire less able to respond to harsch impact the loads on the suspension parts will increase significantly requiring them to be beefed up resulting in more unsprung mass and perhaps more importantly a bigger disruption of the airflow to the rear as a result of these larger suspension parts.
2 Say the part of the tyre deflects during an impact with a kerb is about 1 kg. That is much lighter than the combination of the complete tyre, the wheel and part of the suspension which would have to move if the tire sidewall doesn't. This means that the ratio between sprung and unsprung mass deteriorates. If one likes to read up on it, see the wiki link:
https://en.wikipedia.org/wiki/Unsprung_mass
This link I shared earlier in the thread from James Allison in 2020 on his view to the move of lower side walls on a F1 car.
https://www.racefans.net/2020/06/04/...lower-allison/
That was for the current 720mm 18''. The current proposal for 2026 goes even a step further in this direction regarding lower side wall height.
He also reaffirms the whole marketing and "road relevance" angle of the 18'' tires.
One of the reasons I get a bit frustrated by people on here criticising the FIA while often letting the FOM of the hook. To me the FIA has proven to be a better guardian of the sport in a pure sense. They are certainly not without fault, however the FOM, the manufacturers and Pirelli with there silly insistence on 18'', if left alone, I believe would go quite far to compromise the integrity of the sport to suit their commercial and business interests. For example the entity pushing for lower weight has been the FIA, the trio above does not care much.
I probably won't go into it much further.