I noticed a lot of people confuse "realistic" immortality (which is presumably achievable through advances in medicine, but does not imply an infinite life span - only a very very long one and the absence of mortal disease) with forced (mandatory) immortality (which means you are sentenced to live forever, you can't die no matter what you do). I am perpetually surprised how few people realize that only the first kind…
Talking about the extremes of things - such as this - is a useful way of thinking about the subject. After all, physicists posit perfectly spherical cows in a vacuum, why can't we posit something at the end of the spectrum of possibility?
"A farmer is having trouble with the milk production of his cows, and winds up engaging an astrophysicist to write a report. Three weeks later, he opens the report to find it begins: 'Let us consider a spherically symmetric cow, emitting milk equally in all directions..."