Good question
This is the connundrum that everyone is pondering these days. The correct answer is no one knows & won't know even after ther results of the first decision board is released 2-4 weeks after the first set of interviews. It can/may/will vary as the pool of candidates change each month.
An argument could be made as you have proposed that a higher selection rate will occur since selection for interview was based upon "best qualified". However, the variety of folks who are being called will reflect (IMHO) a cross section of experience (type/no type, 121, 135, 91) & background (mil vs. civilian). Does this make it no longer a "best qualified"? The argument could be it does. However, Southwest is the one to determine "best qualified" & once it does it could easily choose folks from a variety of backgrounds & experiences but still keep the selection rate at historical levels. But no one can't be sure either that 90% of the folks would be selected!!!! Wouldn't that be great but again I have NO CLUE!!! The candidates themselves will determine that.
As you can see the speculation can lead you to any conclusion you may wish to have. Attitude & methods to gauge that will be critical cogs in the decision matrix as the interview process & DB process plays itself out. What one needs to spend the majority of time doing in preparing for the chance to interview is to make sure all the things one can control are done in advance in order for each applicant to be relaxed & come across as themselves.
Selection rates aren't something any of us know or can control but making sure we have copies of our HS, college (if applicable) graduation certificates, driver's license, addresses of previous employees, 3 LORs, checked your credit ratings/background, driver's background, tickets, etc. & other requirements for the interview are the things I would be most concerned about.
SWA will no doubtedly miss a few on the interview process (as we have in the past) & not select some highly qualified folks. It is inevitable & how folks respond to that dilemma will set the tone on whether they will be stronger candidates the next time through (yes SWA has hired folks on their 2nd, 3rd & 4th try).
It is nice speculation on selection rates with this "new" system in place & the quality of candidates we have but so is speculating how many angels dance on the head of a pin!!!! I'm not trying to belittle your question but as I've suggested to many others put energies into the things you can control & leave the rest to a "higher power". Cheers & have a great weekend,