To add to what others have said, there's nothing abnormal about the leaves turning dark colors before they drop, and it is entirely normal for them to fall at this time of year.
I would be concerned about the plant getting so tall that the leaves no longer receive sunlight. This is more or less an inevitable outcome for this plant indoors if it manages to grow as tall as yours. Keep up the care that you've been giving for the last 20 years, because it seems to have worked out very well, but bear in mind that this growth cannot go on indefinitely indoors. Your plant is very close to the limit in terms of how tall it can get before it begins to fail in that situation. It is a tree, after all.
You can restart it from a short-stemmed cutting (best done in spring at the start of the growth season) if you really are intent on keeping it alive (instructions upon request). But I would just celebrate its majesty in the moment for as long as it lasts.
