> I've long since run with the notion that if you must have Smart TV features, do it with an external box
I'd go farther. I want everything to be external boxes. Right now I'm looking at having to get a new A/V receiver because mine cannot handle 4K video. It does all the audio stuff I need just fine.
If video switching and audio processing was handled by separate boxes, I'd just be looking at changing out the video switching box.
Now suppose that later I decide I would like to be able to control my receiver with Alexa or Google Home. As it is now that will be another "replace the whole damn receiver" moment. If it were all separate boxes, the preamps, amps, surround sound processor, and radio receiver would all be separate components, controlled by a controller box. If I decide I want Alexa, I'd just buy a new Alexa compatible controller box.
Or I decide I want to upgrade from 5 channels to 7 channels...buy a couple more amp boxes and speakers for the new channels (and might also need a new surround sound processor box).
Basically, take a block diagram of a complete full featured home theater system, and make every block in that a separate box so I can (1) just buy the boxes I need for my system, and (2) upgrade a box at a time.
That said, I doubt TV makers will ever stop primarily making Smart TVs. A Smart TV can function as the only component of a home theater system. Someone moving into their first apartment after school, say, wanting to start putting together their entertainment system can start with a Smart TV, stick it on their home internet, and then use the Netflix/Prime/Disney+/etc app on the TV to watch movies.
That Smart TV probably has a few HDMI in ports, so they can also use it with the cable box if they have cable, and with their gaming consoles.
With a dumb TV they'd need the TV, and they would need an A/V receiver, and speakers, and something to run the Netflix/etc. apps. Buying all that at once might be a budget buster for someone starting out.
By making the TVs smart, the TV makers greatly increase the chances that a TV will be the first component bought for a new home theater setup, and probably also increases the chances that that first TV will be a big screen, high resolution, high quality display model.