I recently purchased a new 4K smart TV. The funny part happened when I had my first potential purchaser for my old smart TV came to look. The old TV needed a software update. "I have never used the smart features, preferring ROKU" was my answer to be unprepared for the demo. It is true. With a surround system, allowing HDMI passthrough/routing I prefer to have my smart features outside of the TV. As I read through reviews of new 4K TV's before purchasing, I realized that many operational issues that I observed on my sibling's TV (Sony with an android based OS) requiring periodic reboots are related to the OS and apps. Additionally, others were concerned about viewer monitoring via the google based OS. Personally I care less about voice and motion controls, and more about the only three smart apps I really use... Netflix, Amazon, and Plex.... (I also have DirectTV).... so another ROKU was part of the plan.
When I set up the new TV, I ignored messages to initialize via a google account, and went manual. I purchased a new 4K Roku the next day. It works as well as the prior ROKU. Set-up was a breeze with a wireless network. Most surprising was the upgraded app friendliness. As an example, Netflix detected a new 4K set-up and asked me if I wanted to upgrade my plan to 4K for a couple more dollars a month. Some people might find that unwanted, but for me, I had already planned on it anyway... so it was super convenient to hit "yes". In any case, navigation is the simple and the same for all ROKU's. And for me, I kept the old remote (which works just fine) because it has the Netflix and Amazon hot buttons (unlike the remote that came with the new unit).
ROKU provides an alternative op system for smart features that I appreciate and find simpler than navigating my TV's own smart features. ROKU can be pared down to the essentials, and for me reduces the number of remotes I need, along with making things less complex. For those reasons, ROKU can be great for current owners of smart TV's.