There's also the vinculum thing: Borg Drones might live forever independently of the fate of their bodies.
I don't see much reason to disbelieve in the transfer of consciousness. It can be done in part in Trek; it could well be done in whole as well, there being no fundamental showstoppers. If somebody wants to get philosophical about it, fine, they can opt not to live forever. Others would choose otherwise.
Immortality aside, was there ever any followup to Scotty's transporter stasis? Such a tech could literally be standard medical emergency protocol for when death is seconds away, & no cure is forthcoming. Lock them up in a pattern buffer until the necessary administrations are available, even if that takes years
Initially we were let to understand that staying in the transporter for a long time would be bad for your health. But VOY "Counterpoint" shows people being stored in phased matter form for days, without any mention of ill effects from this prolonged use.
This is different from what Scotty did, though, because we aren't explicitly told that the people hiding inside the transporter would be "on hold". The point is to make them aphysical so that the bad guys can't find them, not to prolong their lives or anything; perhaps they are alive and conscious inside the beam (and pretty thirsty and hungry when they get out) like transporter users normally are?
Trek has separate means of prolonging life through putting it on hold. VOY "One" shows the technology that has replaced cryogenics as the means of inducing stasis. It appears to stop time altogether so that the bodies won't even be affected by radiation. The same tech might be in use in mortuaries, as the box which imprisons Odo in "Invasive Procedures" is also called stasis and similarly has time-stopping but harmless effects on our favorite biological sample.
I guess all this still touches upon immortality: as long as you don't die, it doesn't necessarily matter that you don't live, either...
Timo Saloniemi