I think BMW didn't fit a temp gauge because the temperature varies a lot.
I guess it's quite hard for them to know how hot or cool in this case the car is running when they have it in for work/service. Therefore it's important people check their running temps on diesels. A decent Indy will change them both for just over £200.
Cool running is certainly going to produce dpf issues eventually. You may get it warm enough on occasions to prolong the impending failure but once you fall below 80c you are close to issues.
Before my stats were changed I would find coasting, cold weather and heaters all made my temperature drop. All 3 made it worse. I had been running at around 83c for the last year perhaps the main stat failed some time ago, then the egr gave up and I struggled to get above 76c.
Before at 83c my dpf regenerated every tank, now at 92c, coasting, cold or heaters don't affect the temp at all, I expect the book guideline of a regen every 1,200 miles as there should be enough passive heat to help burn some carbon.
Another advantage is I think I get slightly more power as before my car would limit red line by those white line indicators on the rev counter when cool. Now at 92c it seems to rev slightly higher.