For starters, I don't know if you've seen the previous thread on the topic, but I think it covered the matter pretty well.
https://forum.atlas-games.com/t/long-lived-sahirs/7492/1
I've never run any detailed sims of Sahir aging and, in fact, lack the math skills to do so, but I will point out that a character doesn't neccessarily need to fail a Crisis check to die. It happens more or less automatically once the character acquired a Decrepitude Score of 5 or greater.
Also, unlike Hermetic Magi, sahir must support themselves (at least one Season/year) as well as making their annual contribution of Vis/Time, etc in service to the suhhar - the time they have to devote to vanity projects is correspondingly less than their opponents within the Order of Hermes.
Finally, it strikes me as mildly sad to think of the aged Sahirs, frantically trying to extend their lives a few more measley years by the sort of treadmill-like mechanism described. Especially when their are so many other ways to achieve immortality. I know players are generally dismissive of Might-granting methods of immortality, but given the suhhars' facility at acquiring vis it seems like a lateral move in comparison to the alternative.
PS. while the Sahir tradition has no equivalent to Final Twilight, it would be a mistake, I think to ignore the negative effects of cumulative al-Iksirs and the corruption points gained automatically with increased Warping. While not necessarily fatal, the resultant flaws can potentiall prove quite debilitating.