I'm certainly not speaking for all magi. However, even within the most senior magi there will be those who decide that death and its more definitive afterlife may well be preferential. For the Catholics there are certainly many things they can do to absolve themselves of a lifetime or two of magic-use prior to shrugging off their mortal coil. Death in this case is a much surer ticket into Heaven than Final Twilight is.
That said, I think the number that will become more cautious and thus eventually run out of longevity would be more. However, there may well be a similar number in this group who have no desire to experience death and who thus opt to Final Twilight when they realise they can't keep the longevity rituals going.
What effect this has on the stats (if any) probably depends mostly on your saga and the kinds of senior magi in it.