Galaxy S9 originally came with samsung experience OS and was updated to OneUI. As a device gets more updates and change in whole OS overhaul which happened with OneUI it gets difficult to provide updates bug free and maintain same performance. The phones released with OneUI out of the box are easier to promise 3 yr update from now on since no major OS overhaul will happen for few years to come. If OneUI updated to OneUI 3.0 it will bring more issues rather than good features and performance improvements.
Your fault here, it was launched with Android 8 and got two major update as promised by samsung, it will get another year of security updates. Now company can sell phone till any time. Normally they reduces the price every year so it becomes mid range price by second or third year of its selling.
