Is my idea for a Sci-Fi book any good, and how could I improve it?

Here is my basic idea in a nutshell:

It’s the future, and humans have invented an injection that makes pople immortal, as long as they top up every 3 months. With the injection there is no natural death or death from disease, but you can still be killed in an accident or by a gun.

The injection is owned by a large corporation that also owns most of the internet, tv, as well as many many other things. from mining to real estate.

The corporation charges a small fee for the injection, the equivalent of $20 in todays money. You can go to a hospital to have it done, you can buy it illegally on the street, or you can even get it from vending machines. there are even pay as you go plans, and more.

The story, will have a moral lesson about greed and power. as well as touching on oppression, corruption, and death.

If you dont have the injection on time you start to age, and you can not reverse it. The punishment for all crimes is having your injections held back, causing you to age and become closer to death. Jail does not work anymore because you cant punish an immortal by taking his freedom away for a temporary period, and permanent jail is impractical.

The world is over populated, and food is now expensive but there is still a good supply. Those who used to die from hunger and disease in 3rd world countries are now slaves of the corporation.

In this world, Life is very cheap to the corporation, but the world is obcessed with living and value their lives and the injection above everything.