You don’t have to replace the entire TV. There’s no reason you can’t plug a Roku or similar device into a smart TV once the smart features become obsolete, although it does kind of defeat the purpose of a smart TV.
I have a Google TV from circa 2010, and I suspect that’s what Melbourne has. This was Google’s original smart TV platform that predates Android TV, Chromecast, etc. It was developed in partnership with Intel, and therefore runs a different version of Android that runs on an Intel CPU as opposed to the ARM CPU in pretty much every other Android device. Google abandoned the platform several years ago and replaced it with Android TV. So now there are no new updates for my TV, nor and new apps. I’m not sure the Google Play store on it even works anymore. I’m not sure if it ever actually had a proper Amazon app, but it has a built in web browser and I used to be able to just go to amazon.com and watch videos in the browser. Not anymore; if I try that now Amazon tells me my browser is no longer supported. The Netflix app surprisingly still works, and has a better interface than Roku, so I still use that. But for pretty much anything else I have to use a Roku now.