The thing is is that this is language change in action.
Spot the supernumerary ‘is’? Well done. But don’t expect it to go anywhere. As Language Logger Mark Liberman says in this excellent analysis:
In the case of the extra ‘is’ that I’m writing about today, I believe that people are not getting confused, but are producing phrases that are grammatical — in terms of a non-standard grammar.
Agreed. However, I’d describe it in different terms to Liberman. I reckon that the phrase the thing is has become such a stock term that it has lost its internal meaning – instead of a noun + verb phrase, it’s become a simple noun phrase. We need to go the whole hog and re-spell it, I suppose:
thing’iz, n. Matter in hand; problem; key point.
It really does seem like a sort of lexical transitional species. I’m tempted, now, to go on and quote Darwin and Dawkins on transitional species… but I shan’t, because that would be a colossal digression even by Clutterbuck standards.