Smart TV's aren't very smart for long. I have a nearly 5 year old LG set and is largely unusable for the Smart functions. Add a streaming box which is more likely to update the apps for longer (depending on the OS) compared to a Smart TV.
Yes - with a possible exception being TVs that run Android TV (Some Sony sets do) - though IMO external boxes are still better. When All4 arrives on tvOS the Apple TV 4 and Apple TV 4K (if you need UHD) are gong to be a very good answer.
Amazon have also just added frame rate switching support to the Fire TV OS and the Fire TV 3 (4K model) supports it - but third party apps need to be rewritten to take advantage of it.
I'd never buy a TV based on its smart TV app support. The life of a TV is significantly longer than the life of some platforms. We've already seen iPlayer support disappear on a large number of older Smart TVs and DVD/Blu-ray players when the BBC retired older iPlayer back-ends as the platform develops and modernises. Similarly Skype support on some TVs disappeared when MS decided to deprecate it.