Having gone through much of the same thought process as you describe, I trimmed my tree a few years back.* I liked the focus this brought to the work I did enough that I later trimmed again.
A journal-like approach
My database is now focused on my direct lines (ancestors), their children and about four generations of the descendants of those children. As a graphic, it is shaped a little like a Christmas tree.
Occasionally the lines extend a little further (for example, if I am chasing or tracking a little DNA or want to record something about a particular descendant/author/writer).
The software I use allows me to add associates/witnesses and assign roles, so that beyond those in the family tree, I have unrelated individuals in my database.
Different entries/different character
I am not an equal opportunity genealogist, so not every person in my database has been researched to the same extent. I spend more time on what are unproven or conflicted families than I do on other ancestors/families. My interest in associates tends to be even more specific.
*I did back up and save the database before I trimmed. In the new database, I set out to summarize what I was cutting in a comment tag added to the ancestors of those being "trimmed."
UPDATE: Tom writes, "I've never found a reason to trim a tree."
I didn't destroy the file, and probably could have kept the other individuals in a separate dataset rather than a separate file.
In my case, I had drifted further and further from the truck of my tree. When I was conducting work at a FamilyHistory library, I had done a better job of focusing and prioritizing the work, largely because I had to order films (ala, time and money) to further the research. That work had been focused on those more closely related to me. When more wonderful records became available online, I found myself working to solve problems based on readily available records. This meant that I entered more and more extended branches to my family tree, mostly based on "low lying fruit" (BDM and census). That changed when I set up my four or five generation rule of thumb.
Trimming a tree might not be the right decision for everyone, but it was the right move at the right time for me.