IPv6 is a complicated mess because it was designed by enterprise networking people. Everything in networking is always more complex than it needs to be. Always. Network engineers love to overthink everything and they've never heard of YAGNI.
In hardware, if you can reasonably foresee the need for some feature in the future (where "future" could even be > 10 years), and adding it now doesn't blow out your cost budget, you should probably add it.
The cost of being wrong in the direction of excess complexity is higher than the cost of being wrong the other way. Layers are added, never removed. A deficiency can be fixed by adding something. A misfeature or ugly hack is a boat anchor we lug around for eternity.
My guess is what you consider unused complexity is just something you don't see/deal with on a day to day basis so you wonder why it's there. This doesn't make it useless/inefficient it just means you don't use it (or don't interact with it directly even if it is used somewhere in the network path)