I'm sure they're going to expand it with time to other apps as well.
I can do that as well, but it is stuck on that specific TV, where instead I have plex on many devices and would be able to watch it anywhere. Also, $5/month isn't for this feature alone, it's one of the options for Plex Pass in general. If you don't find the features of Plex Pass enticing or necessary for your use, no one is forcing you to pay for it.
Again, $5/month (or annual or lifetime subs - heck, years back it was possible to get a lifetime sub for $50 or less) is for Plex Pass, which includes a variety of advanced features and options. There's nothing to "get" here. It's a new feature they're working on that will be available to Plex Pass subscribers.
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Essentially yes, but it really depends on the OTA signal where you live. That can be highly variable. It would be best to research more of the OTA side before going this route.
Myself, I have a decent OTA setup at the office and at home (haven't had cable for years). Mainly did it to watch the World Cup a few years back, as well as some local stations the odd time (I installed it myself and the hardware is relatively inexpensive). Useful also in case someone wants to catch the odd awards show or some random sporting events throughout the year.