I kind of get what you mean about the over familiarity of it and the passions reducing but that's a life thing! Work, kids, grandkids all take our time and energy which gives us less time for our partner. I guess the passion becomes less important when you have a list as long as your arm to do every day and you just need to survive!
DH and I have been together for over 30 years now and yes it's not like it used to be. But some ill health, age etc have definitely impacted our former nights of passion but that doesn't mean it has disappeared completely.
For me, it's important to look back to those early days and remember what it was you used to love about your partner - why did you want to rip off his clothes and also why did he want to rip off yours?! Long term relationships need work in order to work so there is nothing wrong with being aware of this kind of trough that you may end up in from time to time. It's important to then do something to pull yourselves back up to a peak again.
A weekend away, holiday, date night. Reminiscing about younger days and spending time with other people can definitely stir up some passion. Getting glimpses of your partner with other people, for me, reminds me what a lovely guy he is and why I fell in love with him. His jokes and kindness to other people around him. Those little glances he gives me across the room and me to him.
It's never a done deal, marriage or long term relationships - it's always a work in progress, a bit like fitness. You can't go to the gym for 2 years and then never again for 18 months and still expect to do the splits. If there are no issues other than familiarity, life challenges and a bit of melancholy then it's worth the effort, I think.