Honestly, it will be what YOU make it. Every couple is going to be different, for some their sex life will get worse long-term and for some it will get better. But it's all up to YOU.
For us, it's only gotten better. Like others have mentioned, there is now a level of comfort that wasn't there before - we can laugh, talk, deal with something if it goes wrong, or whatever else during sex. Plus, since we now know each other's preferences we can push all the right buttons. Also, now we know each other's sounds better too, and other cues - so we can adjust timing & pace to match what the other person needs just from listening to them. Plus, like Gunsmoke said - that intense emotional attachment before, during & then after sex is just out of this world. Even though we loved each other before, with more time, the feeling just gets deeper.
Another thing that makes it better the longer we're together is that we both feel safer expressing ourselves and our fantasies. We're now comfortable enough to keep experimenting with new things to see if we like them or not. As we experiment, we find we love some new things, and really dislike other new things. The testing is half the fun! When something doesn't work out like we hoped, laughing about it and moving on is a real bonding experience.