The WebOS apps on my LG TV are actually pretty good and seem to be updated fairly regularly. The only notable exception that I have noticed is Twitch.
I have a 3rd gen Apple TV for stuff that will only work with Airplay but honestly I find that can be pretty fiddly at times.
It is surprising in a way how Apple was pretty early out of the gate with Airplay on Apple TV and had it pretty right early on, but since then they've conceded so much space to TV manufacturers, Amazon and Google.
I've generally believed that the killer app is being able to pick something on your phone and have it appear on your TV, having a big dumb interface on a massive screen so far away from you is just... eh.
That experience becomes a lot faster when it is just built into the TV and can interrupt whatever you're currently looking at, failing that you want something that you can just leave and forget about until you need to do something with it. I don't want another dumb remote that I'm going to misplace somewhere.
I have thought about getting a new Apple TV recently, I already pay for Apple Music and if there was some decent video content that didn't involve James Corden I would potentially contemplate upgrading.
The other point that makes me more dubious about the built in LG apps is how it is reported to just monitor absolutely everything you do, which doesn't sound great.
So I guess that's an answer? It doesn't really have aa much of a place as it did a few years ago. The competitors pushed into a cheaper line of products that seemed to do most of the things people wanted, whilst Apple... made everything more expensive and added some games?