Why is Heaven necessary? Because: evil accumulates. Because *this* world is based on vampirism (life feeds on life)

This is how I see it...

Some people perceive no need for Heaven. They either this this life in this world suffices; or else they want to give back to God their entrance ticket to mortal life - they desire to cease to exist as separate souls, and to become reabsorbed-into the the divine - into the totality. 

But there are ineradicable problems with this life in this world - not matter how ideally things might be arranged; because this is a world where entropy - death - has the upper hand, and wins in the long run with respect to every Being. 

Beings are eternal, and have agency - but in this mortal world, bodies and all physical manifestations are temporary.  

Therefore, over time, there are more and more once-embodied, now dis-embodied, but eternal spirits - and some of these will have made the choice of evil. 

Yes, dead (discarnate) Beings can be replaced by more (incarnate) Beings. Yes, additional creation can keep pace with disease, decline, ageing, death... 

But the problem is that evil accumulates - and that is why the world keeps getting worse. 

(By evil, I mean that which opposes God, and divine creation - which has made that choice.) 

The trouble is that in order to live - life feeds on life. 

And this applies to the spiritual as well as the physical. "Vampirism" is the rule of this world - for all those who are unwilling to accept oblivion - and there are always some of these (and, apparently, more and more). 

What happens is that there are spirits who maintain conscious and agentic (motivated) life by feeding off the "life energies" of other Beings; and some of these are Men. They spiritually vampirize other Men in life, and - unless there is repentance - this continues after death - when they become spirits. So, these evil-spirits of once-incarnate Men accumulate in this world; at least for so long as there are living Men for them to feed-upon. 

Some others are Men who never incarnated - what we term demons. These also maintain their consciousness, energy, power - by consuming others.

What this means is that while God can keep creating, and adding new beings to this world; this also has the side effect of increasing the 'food supply' of demons and evil-men (alive and post-mortal) of the Vampiric type. 

Thus evil accumulates in this-world. 

I think this was understood by the ancient Christian (and other) prophets who foretold "end-time" when evil would have the upper hand (the world was "net-evil"), and where the longer things continued - the worse (more evil, overall) they would get. 

They foresaw that the only way-out was that there must be a "second creation" - one that excluded death and "entropy" - a second creation of pure creation.  

And this is what Jesus Christ made for us with Heaven - that Heaven which we enter via death and resurrection. 

Because at resurrection we (choose to) leave-behind all this is evil, all that opposes divine creation - and we become Beings of pure creation - which is pure love. 

Thus Heaven is "necessary" in the sense that otherwise the world will just keep getting worse and worse; the longer it continues.