In the past I've had issues with stutters caused by not having enough free RAM.
The default launcher seems to use more memory, and I still have a custom leaner launcher installed.
I don't have any issue on the 2021 version.
Anyways, the real reason is that I'm keeping it old just to test if behaviour changes in Dolby Vision playback are caused by the TV or device.
Knowing that the device isn't getting updated, I know the TV updates make a difference.
Nowadays I have no more use for the 2018 stick, as the 2021 is better in every way as long as I use Kodi.