A few ways to look at this:
1) If you are going to have a three meet State series, it probably makes sense to eliminate half of the teams that are going to be eliminated before the final in each of the first two weeks. So for 3A, which generally has between 135 and 140 teams, with 28 qualifying for the State Final, having 84 remaining after the first week is probably the way to go. 72 definitely is not enough, and you should never advance fewer teams from regional to sectional than from sectional to state. People are going to be unhappy this fall; let's get this right in 2021.
2) As far as individuals go, I think I would go for 7 advancing from Regional to Sectional as well, with the additional requirement that they finish in the top half of the field. I'm not sure I agree with the reasons for advancing individuals from Regional to Sectional who will just fill the back of the field. A lot of that happened when 6 of 9 teams advanced out of the regional round.
3) If somehow we could develop permanent cross-country courses around the state that could handle a 250 runner meet, we could eliminate the regional round altogether.
4) How will the IHSA draw the lines? You know there will be a downstate 3A sectional. It'll pick up all of central and southern Illinois. Where does it go next? The south suburbs? Rockford and the far western suburbs? Will the west and northwest suburbs (yes, that's you, DVC, DuKane, WSC, and MSL) end up in the same sectional in the name of geographic balance? Why don't we bring back Ron McGraw to draw the lines for us?