Someone gaining Might =/= True Immortal.
Just gaining Might (Transformed Human) is very fragile and can be undone with a single (Might Stripper) spell RAW, while that same spell only knocks off some temp might points in a True Immortal. Just gaining Might is temporary, though if you are careful that temporary can last a very very long time.
Being able to remove Warping will not make Magi immortal or even immune to age. There is a limit on how high Arts/Abilities can get vs how powerful a Longevity Ritual has to be to sustain them. While being able to remove Warping removes the "275 year from first LR" limit, that limit isn't really the limiting factor. That is because you don't instantly go into Final Twilight when you hit Warping 10 and the 1 point a year from LR won't kick a Twilight off. What the limiting factors are is Lab Total (plus modifiers) and Seasonal Vis.
The longest a Magi can survive is roughly the sum of:
- Lab Total of LR (limited by Seasonal Vis)
- Innate Aging Modifiers x5 (Faerie Blood, Bronze Cord, etc)
- Environment Modifiers x5 (Living Condition, Healthy Feature, Lab Health, etc)
- Variable +/- range for failure(s) of LR
- Time it takes to die of extreme old age with no LR
Magi who get old have to be careful to stay within their Environmental Modifiers the majority of the time since those are calculated annually.
Recasting the LR at the extreme end will be at the point where they can't create a new powerful enough LR due to limits of Lab Total and Seasonal Vis. At that point they might be able to get a few more years by recasting the old one repeatedly but to do that need a ton of Vis.
Most Magi who do not meet a violent end are going to die of failed LR and old age rather than going into Final Twilight.