A separate box allows more consumer choice, which is generally a better experience.

In the life of my last TV (10+ yrs), I've had to switch out that separate box three times. It would have sucked & been way more expensive to have had to replace the TV each time.

Firmware can be updated, sure, but there's the risk of some internal component failing. There's the risk of the services I want to use not being compatible. I'd also prefer to use an operating system I'm familiar with, because, well, I'm familiar with it, rather than some custom firmware from a TV company whose goal is to sell your data, not make a good user experience...

Of course, this ties back to the enshittification of the Internet. Every company is trying to be a data broker now though, because they see it as free passive income.

Regarding the failure of internal components--there are some 'failure' modes which I had not even contemplated previously.

I have a TV that's only about 5-6 years old and has a built in Roku. It mostly works fine, but the built in hardware is simply not fast enough to play some streaming services, specifically some stuff on F1TV. And before anyone asks, it's not a bandwidth problem--I have gigabit fiber and the TV is using ethernet.

Anyway, between that, general UI sluggishness and the proliferation of ads in the Roku interface, I switched to an Apple TV and haven't looked back.