I, personally, don't think grieving is a linear process. My father died in 1987, my sister in 2007 and I still have to stop myself when I want to share something with them or ask for advice. And then, of course, I feel sad. For me XH is similar since who he was for me no longer exists.
When someone was the person you normally went to, talked to, sought solace from and they aren't there anymore, it's sad. Sometimes there is no one else you can talk about some specific thing with. It leaves me feeling very empty and alone. I suppose a person who has found someone else that is their match would not have this issue, but I have yet to find anyone I trust. Or I have yet to trust myself. Couldn't say which, but it's hard to meet people when you work from home and can't get out much. Point being that you probably miss having SOME one that you can talk to or be with. It really would not have to be your x, but that is still your go to in your mind. Being sad when you feel alone is normal.
Is your question more do people ever stop thinking about their x? It depends on the person and situation. Or are you wondering how long before you stop missing your x? Or are you wondering if the empty space your x left can be filled in should you meet someone else at some time?
For myself. 9 years out, 95% of my time XH has no free rent space in my head anymore. It was 2 years to get to tolerable, and probably another year to get to I'm good without him. The time when they are doing that push me, pull you garbage doesn't have any healing in it, you have to wait to be done with what was, because that part of your life is past. Acceptance of what is was key for me.
I do have two kids, so there is a small amount of time I am required to acknowledge his existence, but mostly he is just an annoyance. I don't happen to have anyone, but that is my choice. I do have friends, work, adventures and hobbies which take up a lot of my time. And when I feel lonely or sad, I either listen to sad music and let the feelings wash through, or go find something I like to do and do it.
Don't be afraid of being/feeling alone. Let it be a transitory thing. That way should the right person come along, you will know you are not just looking for something to fill the empty space. JMO, as always.