The difference is that with all the past additions, they have been accretive to the revenue pie. Adding teams made more money for the existing conference members. OU and Texas are better than average SEC teams and bring additional eyeballs. All the west coast teams brought the Big 10 eyes in some huge markets. There isn't a lot left. The ONLY team left for the SEC that wouldn't be dilutive would be Notre Dame or maybe UNC. They already have flagships in all the states with remaining decent programs. The Big 10 can add a bunch of schools like FSU, Virginia, etc. where they currently have no presence.
I see us ending up with 3 major conferences: SEC, Big 10 and Big 12. The fact that there are still some good assets in the ACC will ironically seal its doom. FSU, UNC, Virginia and either Miami or Clemson will get divided up between the SEC and Big 10. The remaining good assets will get gobbled up by the Big 12 - and will fit perfectly. Virginia Tech, Pitt, Duke, either Clemson or Miami, maybe NC State, maybe Louisville. The rest will join has bens like Oregon State and Stanford and wind up in rag tag conferences that are regional.
I think the 3 main conferences will contractually get a lions share of the playoff spots. SEC and Big 10 probably get 3 each. Big 12 gets 2. Leaving 4 at-large. Maybe one spot gets reserved for the pool of also-rans as long as they are in the top-25 or something (and a conference champ).