Desperate times call for desperate measures.
The ACC saw what happened to the PAC - erased from existence. They know that Clemson, FSU, and more will leave eventually. Couldn't risk also being disintegrated, so took a page from the Big12's book - added teams (even if not that great) because "strength in numbers" is worth something to keep the conference name alive.
Similarly, Cal, Stanford are desperate not to be relegated - which will probably be a forever relegation this time. And in the ACC, they can at least brag up Duke, UNC, UVa, BC and the other quality academic schools to be associated with. Same with SMU - this is likely their only and last ever shot to get back to the top level post death penalty. Doesn't matter if they have to borrow $200M for the next decade, they can make it back in the top level. But if they don't make it now, they are probably G5 til the end of time.
As far as Big12 impact of this... we've been assuming that the B1G and SEC would take 6 teams from the ACC eventually, and the remaining 8 would be hard pressed to stay together (media value $20M per?) so the Big12 would be able to grab the best 4 leftovers; effectively killing the ACC. But with the 3 adds, now there would be 11 teams left... that might be enough to cling together even with a lower value media deal?
Its very similar to the PAC situation, but they already have the adds in house, they wouldn't be scrambling to justify San Diego St or Boise like the PAC was. And they have better leadership for sure. So MAYBE they could hang together? Although still would be a lot of pull for a Louisville, Pitt, etc to jump to the Big12. Honestly, I don't think its enough - teams like LV and Pitt and VT would probably like a conference with old BigEast rivals like WV and Cincy (also no geography or academic prestige objections). But at least the ACC is trying to get ahead of it and executing a plan - unlike the PAC which was clueless and leaderless.