>it isn't clear why your lifespan wouldn't be indefinite.

it also isn't clear why it would be, when achieving something not achieved before you end up uncovering undiscovered problems, opportunities, and constraints.

The point is that reliably making it to a few hundred years requires (AFAICT) full understanding and control over all the elementary biological processes. If you already understand and are capable of freely manipulating every primitive in the system it isn't clear what's left to break.

To put this in mechanical terms, once I know how to replace every last component in my car and have the ability to fabricate new parts for the body and frame under what conditions could my car ever be unrepairable?

and my point still remains, once I know how to do something that has never been done is there a chance that some new factor will be revealed by this ability which will somehow constrain my ability to achieve my true goals? As the description of the scenario was of course fictional I decided to describe it as though some constraint unfamiliar to us now still kept things from going onward.

One particular constraint you can imagine in this fictional situation is that psychologically people who live more than 3 centuries start to have a deep burnout of existence, because there no longer seems to be any challenge or newness, causing them to experience extreme depression and psychological illness of various sorts that most people do not experience nowadays with our short lifespans. Thus there are psychological profiles done, if you are around 500 and you are not shown to have hit the psychological end point of your existence you can be allowed one more child, also dependent on how many children you have had before. This however will be your last child, because no rebuilt human has ever managed to escape "age psychosis" after 600 years of age.