I think it will come down to marketing for the Land Cruiser 300... identical dimensions to the 200 platform mean the hard points didn't change, so it will be a progression or variation of the 200 platform. If they change some of the chassis bracing or weld technology... sure, they might call it "new" or say it is a TNGA platform, but I guess we will have to wait to see.
I am with you that I think the LX will move to the new F/F1 platform with the Tundra, Sequoia and others, and become a bit larger. It makes logical sense with what dealers and buyers want, and the new TX coming.
I very much doubt LX will not share a lot with LC. It is not that simple - Tundra and Sequoia are much cheaper vehicles than LC/LX.
People need to remember that LX is first and foremost be a better version of LX, not something else.