So much theorycrafting about making V survive. Like I, personally, don't understand why he has to survive. Endings bring a very powerful message, especially if you do suicide or do corpo slave route or allow Johnny to take over. The dialogs, the messages, the music. It is a very strong message. Yet here we are demanding some "better" ending.
As far we can surmise - V was not meant to stay alive. As is original V, not engram.
But if we so choose to let engram continue to live, instead of Johnny, we are supposed to check couple endings to get more info of why Vs engram can't live in his original body. To answer that question, first we have to understand how soulkiller works and what engram is.
Checking wiki: https://cyberpunk.fandom.com/wiki/Soulkiller
Soulkiller virus is a program capable of creating a digital emulation or copy of a netrunner's mind, utilizing an advanced matrix recorder storing it in a huge database
So basically a kind of a "copy" of what you were at some point of time.
Meaning that engram is a "copy" of who you where, no you. Meaning that Saburo left copies of himself in each Arasaka tower over the world just in case and at some point will be updating each of them up to date(We can think of this because of Arasaka ending)
Now, onto why "immortality" is kinda possible:
Once again, we read wiki:
The Relic is a series of Arasaka biochips allowing the storage and manifested reading of digitized human psyches known as engrams. At least two separate versions of the biochip prototype were developed by 2077, according to the summarized internal report assembled by the former director of the...cyberpunk.fandom.com
And this gives us even newer piece of info - engram without relic should not even be self-aware. This raises a shit ton of questions I don't have any answers. Especially how Alt Cunningham is alive AI that has huge computing power and wants to expand it even more by assimilating even more engrams. Probably because some things behind that "Black wall" are giving her a hard time. But not the point of relevance here.
But back to topic, only version 2 of relic, one V got in himself, allows to "reincarnate". To a player it might seem like it is straight up reincarnation or plain moving from one body to another. But it is not, why? Because unlike "Cortial Stack" from "Altered Carbon" that allows to move you from one body to any other, Soulkiller or engram with relic v2 does not do it. Meaning that if you, or in our case V, dies, that engram that becomes self-aware - who the fuck is that? Like seriously? I don't have an answer.
Why V can't go back to his body if we wipe Johnny's engram from him? Because his engram is a copy of V after it was altered by Johnny. So well, even a copy of V is still affected by what Johnny was. If we listen to their dialogs, especially in Arasaka or Panams endings, we will get to know that, firstly Relic does reconfiguration of Vs body. Meaning that it adjusts host body to engram on chip. Why this happened? Because gonk's body got a bullet in there and died with chip inside, so chip still having some power in it went to work. Being experimental and not fully working thing all of this is unexpected to even the author of this Relic. This leads us to a valid thought that if Hellman or anyone else creates another copy of this same Relic and fills it with Vs engram, then we take some recently dead body and damage brain(while planting chip before) or maybe just take dead body and that would be enough. This will solve our ending problem.
Real question is, is it worth it?
It boils down to this, for a game that is built upon survival, where you the player are fighting hard because you're under the impression that you need to survive, the fact that you can't is a failure to some. You're geared up to fail. It goes against the whole journey, the reasoning behind it all.
Why sell me a game where the baseline of the plot is to 'survive' but in reality, I can't? And not because I didn't try hard enough, or didn't give it my all. But because the game refuses to reward me - the player - for my effort.