If you've just released a pretty good product but come up with an improvement just a short while later, should you release it immediately, or wait for some time to pass? The advantage of going ahead is that you can keep up with competitors who might have released products with new features and capabilities since your launch, and you get to create a whole new hype cycle. The downside of course is the backlash from buyers who picked up your earlier product and would feel let down or even betrayed by such rapid forced obsolescence. That's exactly the position OnePlus[1] was in, and from what the company has said, it didn't hesitate at all to launch the OnePlus 5T[2] just five months after the OnePlus 5[3].
This is a company that has built its identity on delivering more than the big guns can, and that too at lower prices. Samsung[4] and LG both had the same idea earlier this year - make phone screens taller instead of increasing their size proportionately, and that way you can stretch them without the awkwardness of an overgrown phablet. However, Samsung and LG are gigantic multinational corporations that manufacture their own displays. It took a while for third-party suppliers to catch up, but Vivo[5], Oppo[6] and Honor[7] are already shipping 18:9 models. OnePlus just couldn't afford to not be playing on the same level. That's why the OnePlus 5 is now on its way out despite being launched with much fanfare only five months ago.
Very little else about the OnePlus 5T is new[8] - its processor and nearly all of its hardware as well...