Three general comments:

1. I view "miles per day" more in the context of budgeting time for a trip, than a particular goal I am trying for on the trip. Two recent examples:
- I'm just finishing a ride this July from Abilene to Minot. Prior to the ride, I wanted a rough estimate of time to allow to budget my vacation time, return trip plans, potential adjustments, etc.
- I did a ride from Deadhorse to Ushuaia where the seasons at either end mattered as well as rainy periods in between. To create a rough plan, it was useful to have overall distances to work rough calendars.

2. The metric I pay a bit more attention is "hours per day" since it gives me a flavor of the ride. For many years I had a rough planning approximation of 10 mph including all my stops. I'm a little slower now. However it gave me a feel that an 80 mile per day ride I would be on the road ~7am to ~3pm most days with some longer and some shorter. Taking rest days would also push that average higher than 8 hours on the remaining days.

Once I'm on the trip, I'll do what I need to keep in that rough mileage budget and that then means throwing more time at it.

3. It has changed over time as I've gotten older. My average speed has gone down slightly, but at least as much is a preference for shorter days and taking an occasional rest days. In my 20s, I had more of an idea of cycling from early morning to late afternoon. In my 50s, I'd be happier stopping midday. This has been the biggest factor affecting miles per day. To give an example: My first trip across the US was 95 miles/day. Five years later I crossed Canada doing 85 miles/day. Four years later my second trip across US 75 miles/day and around Australia at 55 miles/day. Five years later crossing Russia at ~55 miles/day. Alaska to Argentina was ~30 miles/day with averages pulled down by time spent not on the bike e.g. a few months break here and there. Time I was actually riding would be closer to 50 miles/day.

So I think of miles per day as more describing the type of trip (e.g. a trans- America race vs a mix of biking and other explorations). In that preferences are going to differ between individuals and over time. Unless it is explicitly part of the exercise (e.g. Tour Divide race), I see the metric of more of a planning/characterization aspect than a competition.