I don't want to completely rule this out, but I'm not a fan of these high density connectors.
I know Intel Edison uses one. I know others have gone down this path, some
more successfully than
others have tried.
Some of my reservation is a concern this doesn't fit PJRC's business model. Teensy has never been about making a comprehensive collection of "shields", or whatever you'd call such a plug-in peripheral board. If I were to start down this path, odds are very likely the dev time that I currently put into creating more advanced software (OctoWS2811, Audio, PulsePosition, ILI9341_t3, SD_t3, SerialFlash, Biopotential, new USB types, FreqMeasureMulti, etc) would almost certainly be consumed by making and supporting lots of hardware. Many other companies in this "maker market" do this, and indeed it seems to be far more profitable than PJRC's approach. My decision making is really not about what will make the most money or conquer more of the market, but rather how to maximize (or at least sustain and continue to fund) my ongoing efforts to develop awesome stuff.
I'm also reluctant to create yet another plug-in form factor. Teensy is probably among the more successful "unofficial" Arduino compatible boards, at least in terms of actual usage in real projects. But so far, to the best of my knowledge, Teensy's pinout & form-factor has yet to be adopted by any other board. Even
Arduino Zero clones that could have benefited from Onehorses's many "appallingly small" peripherals haven't been swayed to align their power and signals similarly.
So to even begin to consider this, I'd really want to be interchangeable with an already-established format. (hint, hint... to anyone who wants to persuade me....)
Cost is also a concern. I've always tried very hard to keep Teensy affordable. These connectors vary considerably in cost. Some of the cheaper ones may be designed for only a very low number of "cycles".
PCB real estate is also a possible issue. There are a lot of possible ways to use the space. One space-hogging idea I have in mind involves using the right hand side for the same SD socket we currently put onto the audio board and Wiz820 adaptor, with wiring to the native SDIO pins on this K26 chip. I also really want a convenient way for people to connect to the other USB port (the 480 Mbit/sec one), which probably will end up being a location to solder a 4 or 5 pin header that mates with the common USB-to-motherboard cables made for PCs. These, plus the large 13 mm chip and other stuff required all add up to not much PCB space left over.