I don't think there's going to be a mixed registry, or even a registry on the horizon. At least in the somewhat near-term.
People who send their stuff to SGC obviously don't really care about a registry to begin with. They just want accurate-ish grades, a quicker turnaround, and semi-attractive holders.
I think 1st things 1st, is the search engine for the pop report needs to be cleaned up. Just looking through the T206's while submitting, there can be multiple choices for the exact same card, while not even offering a choice on the various back variations within each brand that has them.
Can be even more confusing with many of the more obscure tobacco sets.
I don't know how much money PSA paid for SGC, but if most that was said is to be believed, hopefully there's language in the contract that forbids PSA from disappearing the brand or even many of it's practices from existence.
I've also just sent in 21 cards to SGC that I've been procrastinating on for awhile. Nothing to knock your socks off, but the announcement kind of motivated me to get it done. Figured I'd already dragged my ass on them for awhile...didn't want to give PSA 6 more months to drag their ass on them on top of that.
I have a hard time believing PSA spent a lot of money just to disappear SGC and alienate a load of vintage collectors in the process. Many who have no interest in losing contact with their cards for months (much, much longer in the not so distant past) on end, and likely wouldn't get a lot of their cards graded otherwise.
Whatever market share SGC has...PSA isn't going to just get it, simply by doing things the way PSA has always done things. SGC got most of that market share by doing certain things different from PSA to begin with.
A lot of collectors will just as soon keep most of their lower grade/value stuff raw, or in whatever holders they already reside in.