AV Receiver Upgraded

I have a Denon AVR-4603  Surround Sound Receiver for the past 13 years,  and it is still working fine, no issue at all. But the question is: 
Why should I upgrade to a new AV receiver such as Denon 4700 or Marantz SR6015, what am I getting? Why would you upgrade?
And Yes I do have the Sony 77" OLED TV.

Unless there’s a newer feature on the 4700 you need or want (4K, streaming, etc.), don’t bother and just continue enjoying your 4603. 
Looks to be an awesome unit, highly respected sound quality, the cons were only about it's complexity, which I have to assume you have mastered.

I would only think about the speakers, fully satisfied? or some improvement possible?

After 13 years, I would disconnect everything, clean all jacks, connectors, back together, there is the potential to recapture ____? 

I use the Receiver for only for home theater. The speakers are Totem Acoustic Rainmaker plus two Definitive Technology cs2000 small subs. The receiver is not connected to the internet, so video streaming is done directly through the the Sony 77"  OLED  4K TV or Apple 4K TV or Amazon Firestick. Having this set up is why I question the reason to upgrade the receiver, seeing that only audio is routed through the receiver. Does that make sense to you? Am I missing anything in not streaming video through the receiver?

Thank you.

If you want a Receiver that is reliable & also sounds good, look for Yamaha, in particularly the Yamaha Aventage RX-A2080 which is a 9.2 channel receiver.

Don't go by the Denon hype, trust me I had one, it sounded broke for 2 channel music listening.
The main reason to upgrade the AVR would be room correction technology.  I was in a similar position a year ago and replacing my 15 year old Pioneer Elite AVR with an Anthem MX520 was a significant upgrade.  I had compared it to the Yamaha and I thought that the Anthem was much more immersive.  And then, even without ARC, there has been significant improvement in DACs in 13 years, which is an eternity in Digital.  I was happy that I upgraded