If you are happy with what you have, and it is causing no technical issues, stick with it. Only reason to add a Roku would be for apps the TV doesn't provide. If down the road you start experiencing tech issues or interminable slowdowns, you can always buy the current Roku model.
Agreed with Scott.. if the TV gives you all that you need, then I'd stick with it. I know some Smart GUIs aren't very friendly compared to Roku's, so if you can't deal with the interface when streaming, get a Roku. For $50, you can get a 4k Roku option down the road.
Yeah, I got some Rokus. https://youtu.be/d1CSEeqWl10
I've been a Roku user for years and highly recommend it to my friends and family. I purchased a new 65" TCL Roku TV on Thanksgiving for my living room and it really is the best of both worlds. If you're going to buy a new TV I recommend going with TCL. I'm so impressed with their quality that I'm actually thinking of buying another one for my office.
I will admit though that I spent the better part of a year researching TCL before I bought a new TV and I do recommend researching a bit before buying based on your needs. I also recommend buying a sound bar with the TV because the newer TV's are so thin that the built in speakers aren't as wide as some of the bulky TV's. I bought one with the TV. Basically feel like I have a mini theater now. Knows more and more TV makers are signing up with Roku, but as someone who never purchased a TCL TV before, it's pretty much the best TV I've seen in terms of picture quality and smoothness. Even though I got the 65" the TV isn't bulky and set up took less than a half hour.
The only downside was the Roku servers were down on Thanksgiving night so I had to wait until the early morning on black Friday to get the TV activated, but once the outage was lifted it was smooth. Good luck.