Good question on the AC vs DC point - I do not have a factual answer for that one.
One thing I do know is that back in the old days, homes got dirtier faster then vs now, often due to draftier construction, no/fewer central HVAC systems, more open windows, more open land/fields resulting in more wind-blown dirt, more fabrics were linty cotton/wool, often more kids running in and out (desire to play outside), etc. This meant more vacuuming to keep the house clean.
It is possible that vacuuming was a ritual for the original owner, like my mom was with her D50. Maybe they had pets or allergies.
My mom always kept a clean, tidy home and us kids did have allergies but no pets. The groove in my mom's D50's commutator (from 1984) was half way through the bars in the year 2020 when I restored the machine for her.
She had used the D50 once a week for an hour or two until maybe around 1995 when she went to every 2 weeks. She had it rebuilt in 1984 or so. She is still using the D50 to this day, with its armature with a 1983 date code on it. I have a new armature and field for her when the commutator finally gets cut in half.
If it happens... She only vacuums once every 3 weeks, but her house (same one!) is still always clean and tidy.
BTW, I hope you got the front bearing as well, as it takes the most stress and dirt being close to the fan.