Things could have probably been different if OnePlus would have said these two smartphones will stay on Android Marshmallow in the first place. Some of this may be due to the company using an older SoC for a smartphone they just released (the OnePlus X in this case), while they say others are because of their premature software team at the time (the OnePlus 2 in this case).
OnePlus's Community Manager and Head of Product used this Q&A as a way to answer a lot of commonly asked questions regarding software support for the 3 and 3T, and while the two discussed a lot of details during this talk, the biggest news to come out of it is that OnePlus will not be pushing out any major software updates for the OnePlus 3 or 3T following Android O. Some hardware-reliant features found on the OnePlus 5, like the Reading Mode and DCI-P3 color gamut output, won't get passed onto the OnePlus 3 and OnePlus 3T - the component gap is real, folks. However, the company remains committed to providing security updates for the foreseeable future.
After the OnePlus 2's very lukewarm response, there was much rejoicing when the OnePlus 3, and later the OnePlus 3T, turned out to be a return to form for OnePlus. "After the OP3/3T have been updated to Android O, we'll start moving the Open Beta Program from OP3/3T to OP5". It's worth noting that OnePlus has been pushing continuous updates through the beta program to test the new features before pushing a public release.
This is why it doesn't really come as a surprise to learn that OnePlus will be done with the OnePlus 3 and 3T after the Android O update.