It comes down to the history of the seats.
Originally, in the Model X 2012 unveil, Tesla showed a "what would you do with all this space" image of a folded seven-seater Model X. I believe the prototype of the time also reflected this. It had similar monoposts seats that eventually shipped in the second row, but they are folding.
As late as in 2014, Tesla sent an email to Model X reservation holders confirming the seven seater will have fully folding seats. It sounded like a bit odd reassurance at that stage, since nobody in public seemed to widely expect it to not be folding. But it suggested, I guess, that the topic was hot in the minds of Tesla, perhaps due to some early issues.
When Model X opened for Signature orders in late 2015, only the seven seater was offered - and it turned out, no folding seats, despite hopeful (and alas, misguided) speculation based on early images on how those static looking seats could be made to fold... There was a fairly significant outcry and some notable Model X reservation holders dropped out of the race so to speak.
It seems it was stunning enough as turn of events for Tesla to make the sudden move of adding a six seater in the Design Studio and retroactively offer it to earlier orders. Again people at first were hopeful that Tesla pulled surprise "captain's chairs" out of their R&D, but that was a misguided hope too. The big news here was of course that the six seater allowed carrying of longer items.
In the end, the six seater is exactly what it looks like: Tesla removed the middle seat, leaving the side seats in their original pushed out locations, and added no arm rests etc. to compensate (meaning the seats are a bit easy to fall out of on one side, and very tight/close to door on other)... Tesla did show a prototype center arm rest for a six seater, but nothing ever came of it.
So, a botchjob, really. A lovely botchjob in some ways, but still, quite crude.
It was speculated the folding of the second row was nixed due to safety or regulatory issues. At the time we were unable to find any current U.S. sold production vehicles with folding second rows that had integrated seatbelts (we did find some Europe models). The unique monopost design and falcon wing mandated in-seat seatbelts were speculated to be a tough nut to crack for crash testing.
Whether or not the five-seater delay of first six+ months and eventually around one year is related is unknown. Tesla did publicize five-seater originally with a picture of the monopost second row, so it is possible the original plans for a five-seater called for the same seats as the seven-seater had, but obviously without folding that became an issue. The long delay with the five-seater, and completely new seating, suggests Tesla designed a completely new seat due to this.
Now, what this all means or does not mean for a potential folding seven-seater? Well, I would say the fact that the five-seater was launched with a completely new bench is disappointing in this sense. Had the five-seater had a folding version of the monoposts, those making it to the seven-seater would have seemed like a logical next step.
Seeing as Tesla went with a budget seat of sorts for the five-seater, it is hard to say anything much about the future of the seven-seater. Folding may or may not come - with Tesla's history at any time, really. The sorry affair of the five-seater deliveries being delayed and having all sorts of issues with backordered and mis-installed (and frankly, miscolored and not very fitting) trunk parts instills very little confidence, though, in that Tesla would have the Model X rear seat game figured out.
Personally, I think out of all these rather unfortunate options, the six-seater is probably the best compromise if you can manage with it. It has probably the most uniform quality and some redeeming factors making it at least a bit special in return for the hassle. Five-seater is the practical cargo carrier, but its part game is still a hit and miss, and arguably its matte design fits a bit poorly in glossy Model X (plus the trunk covers are miscolored)...
As a final tidbit, Tesla recently hired a Volvo interior boss. Volvo is nothing if not practical, so I expect more attention as well as quality to be put into all this down the road, but I am not sure how quickly we can expect results.