People need to get out to eat and drink (and pee and poop). The same may be true for healthcare, they will need medicines, and exercise.
This means that there would still be demand for these things in the real world, and so there are people willing to pay for them... and people willing to be paid for them. So people will still have jobs in the real world.
If what happens to the body in the alternative world happens to the original body - as stated in comments, that may create a loophole that allows to eat and drink inside the alternative world to stay alive. You said that you cannot bring objects from the alternative world, so maybe you cannot take out nutrients from meals, so you get back to the base scenario.
If you choose the loophole scenario, then the bodies are somehow linked, and food in the alternative world is food for the original body too...
By the loophole you can eat something in the alternative world and poop it in the real world.
If the original body is injured I would expect that to be reflected in the body in the alternative world.
A person could sustain itself within an alternative world... and maybe use magic from that alternative word to prolong their lives! Cure any disease!
Some alternative world would be preferred over others. You would one worlds that has prosperity and safety, so they can:
- Give food to everybody.
- Give healthcare to everybody.
- Serve as communication channel.
Why is that decay? it looks like progress to me.
You said you can enter video games too, and if what happens to the body in the alternative world happens to the original body, that means that you could enter – for instance – Minecraft※ and have virtually endless farmable land to produce food to sustain the world. Or find a story with something like the Sustaining Spoon.
※: Minecraft is not 100 years old, no video game is. However, a government could have a long term project to create such worlds. If such extensive and abundant worlds doesn't exist, then people would NOT be too much time in alternative worlds anyway.
If what happens in an alternative world is persistent, then you can transform those worlds. I can imagine conquer efforts to be put to some alternative worlds. For instance, Ambrosia, Aguamiel and any Fountain of Youth are high value targets.
Of course, there is no currency that can be moved from alternative world to another (except by the poop hole)... but you can give a better life to yourself and your people if you have power in one of the alternative worlds.
Once we have a situation when governments are taking control of alternative worlds, we can consider that they become part of the territory. But also means that they are a frontier to all the other countries.
Consider – for example – that France is in control of the world of the Wizard of Oz. Anywhere in the world where people know the tale of the Wizard of Oz, they could try to enter that world, and so they would be stepping in french territory. Even unknowingly.
So, the government will need to have military presence in the alternative worlds to prevent them to be overtaken by opposing factions.
But that is not the only way to attack. You could target the real bodies of the soldiers that are in the alternative worlds. So defense in the real world is also needed. This means that – regardless of the benefits of the alternative worlds – you wouldn't move all the population to the alternative world permanently. This means, that even in this scenario, you will not have cities where everybody is immerse in the alternative worlds.
Long term strategies may include the destruction of copies of a book (or other media) to prevent others from entering. Having a selective group control the access to the only copies of a book, allows to control who is granted access to that story (which is done by allowing them to read).
Of course, you cannot make people forget the stories they already know, but new generations will not know them.
Note on oral tradition:
We have talked about stories being confined to physical media. But there is also oral tradition. If we allow the retell of a story to allow access, then we need to consider what happens as the tale is transformed.
When retold, some details maybe lost or added to the tale causing it to be slightly different. I would expect that that means that it is creating a new story – which is not 100 years old – and so it doesn't grant access – unless retold verbatim.
Note on translations and adaptations:
Since they are derived works, they could be independent to the original. That is consistent with the rule of verbatim reproduction. If we consider that to be the case, then they need to be 100 years old to be accessed. That means that if you access the Never-Ending Story it is not the same as entering La Historia Sin Fin.
The alternative is to allow canons, but there is no reference to say what is considered canon and what not. If we consider something like "whatever the original author consider canon", then them changing mind may cause drastic effects in the alternative worlds, of course it is to be expected that after 100 years most of them will be dead... but if those alternative worlds can prolong one's life that may not be the case.
Replacing the 100 rule with a "death author" rule, means that you may consider to sacrifice an author to create his world (which might be interesting to explore). But as stated, a book that expand on another is creating a new world instead of adding to the original one.