Model 13 had gorgeous design, awesome screen and good cameral. Model 15 lost all of it.
Design became very generic. Same as many other cheap phones without their own face. Boring flat frame, flat screen, rounded corners. Nothing special. Back panel is just a back panel, now without uniq texture.
Screen has lower resolution. It traded it for higher refresh rate. I see difference on small text, but 165HZ - invisible in daily usage.
Camara - simpler hardware, it result is the same at best.
So, at the end of day, you pay 1/3 more for device with worse design , simpler hardware.
It looks like OnPlus took a step in a wrong direction, that no one asked for: from flagship for all to a gaming phone. Phone for gamers needs to have high refresh rate and huge battery, sacrificing screen resolution, camera and comfortable design.