I think it's comes down to this:
- If your "3" does "everything I want it to do", you do not need a "4" or "5."
- If you wish your "3" could do anything else, there may be an app for that on the newer models. Check into it and let that drive your new purchase considerations.
Basically, this is like having that first iPhone before the app store and asking why buy a latter iPhone since that original does "everything I want it to do." Newer iPhones gained all the functionalities of non-stock apps in the first generation. Same here.
For example, one announced new app that is fairly well anticipated is Amazon Prime. You might already have Prime for faster delivery. If so, that app will add a lot of original and canned programming on

TV. The "3" will almost certainly NOT get the Prime App. But the "4" & "5" will. Would you like access to all that Amazon Prime content on your

TV? There's a catalyst to upgrade.
The same applies to any other apps already on the new models but not on the 3. How many desirable apps do you need to desire before you decide to sell or repurpose the "3" and buy the "4" or "5"? That may be a catalyst to evolve "everything I want" into "almost everything I want but I do want...".
On the subject of having to buy a new TV, that's NOT true. You can hook a "4" or "5" to the HDTV you already have and it will simply be hardware capable of doing more working with an output device that maxes out at less. It can't turn a non-4KTV into a 4KTV but it can play whatever your existing TV is capable of displaying to it's maximum.
Nothing wrong with buying hardware capable of "more" than you need right now. I think that's actually pretty normal. New iPhones have hardware bands built into them that some buyers will never use... but the capability is there should their situation change and they want or need those other features. If for some reason, you opt to buy the "5" but hook it to an existing HDTV, you'll just have an

TV capable of delivering higher resolution working with a lower resolution screen. So what. At some point that older TV might conk. When it does, you'll find mostly 4K TVs for sale at the store with 1080p options increasingly fading over time. Buy the replacement TV, drop it into the slot and the "5" will "just work" with it too.