Maybe so, but I never personally asked the TV manufacturers for a smart TV.
Who I think came up with the idea: TV manufacturers.
Selling hardware that has software/firmware, that will become obsolete in 4-8 years, will result in more TVs sold in 4-8 years than would have been.
We just got rid of a TV that we've had since the late 1990s- about 18 years. It was not smart. We just bought a smart TV and I'm betting we won't get more than 8 years out of it.
If I could have gotten the same TV without any smart features and an external Roku, that would have been better long-term, but almost all newer TVs now are smart.
Its important to remember that "TVs" are just "displays whose bundled additional features include at least a TV tuner".
Unless you absolutely need a TV tuner, you can get displays that include neither a TV tuner nor smart features, but they aren't sold as "TVs" because they aren't.
This is simply a fallacy the fact that the software would become obsolete doesn't make the TV obsolete as long as it can still receive a video input and display it - this has nothing to do with the "smart" features of the TV.
I like the interface a lot, but I bought it knowing that at some point, it will not be as cool as it is now. It will probably get in the way, as I'll have to use the Roku remote to browse via the old Roku OS to the port with the box that is the new browseable streaming OS. Maybe the Roku one will get borked by a system update because my hardware is unsupported- I don't know; I hope that doesn't happen, and have had a good experience with my old Roku external box, but it is a possibility several years from now.
Years ago when I bought a TV, never did it cross my mind that "one day the user interface is going to be obsolete".
On my "dumb" TV that is not and never will be connected to the 'net, this feature works fine and has done so for years. On my friend's "smart" TV, the red button feature has been replaced by an "app" that we had to update and fiddle with to get it working, and it won't do anything at all without an internet connection despite the key feature just being a kind of switch between two sub-channels, both of which are served over the usual aerial/cable. You can't even turn this off to get the normal red button back.
A TV manufactured today is very unlikely to last 18 years without some kind of hardware failure. There is a big difference in complexity/points of failure and cost-cutting between 1998 and 2016.
It's hard to believe that TV manufactures are shooting for an 18 year mean lifetime. They might work after 18 years, but it is unlikely.