Also with the you store for me I store for you it would be able to work at a large scale
I can't agree
you store 50gb of data for others you are allowed a credit of 50gb in which others can store for you
to make system fail proof you need to duplicate data and store some additional info too. That means you *must* allow to store more data than you store yourself in the network. Assuming we have only 50% nodes online 24/7 you need to store in about 1:2 ration to make it possible.
A lot could be done with this system everything would be neutral no spam attacks you are essentially paying for space with space. This method does not limit you to a small team.
how can you check that i really have the data not just downloaded, saved hashes and deleted all the files?
Well this is where documentation takes places, again I have documents in which I am working on and documentation is time consuming, if I where to provide all the answers than I am really creating the system for you.
There are ways to achieve what you are obtaining, it is all about being creative.
You would have to answer what system are we building on top of are there premium features? is crypto included? how often are these files being accessed?
This all ties back to checking if nodes really have the data stored, this is the easy part as calling and checking if the data is stored, the question is which way are you going to go about it? The most efficient is through the use of a committee.
If we take a look at other current decentralized storage system most only store while the user is paying, once they stop the file stops being hosted. You would have to assume the same for this system, once a node goes offline their files will also stop being hosted. Clearing up space across the system. So it is to the benefit of nodes staying online, you can put a system also in place in where when nodes host more than needed they are awarded energy/gas which allows them to go offline for a certain period of time while still allowing for their files to be hosted.
If it is a blockchain which utilizes its own crypto on top of it with premium features then we can also have a set of committee/delegates or just any node who has extra storage space they can be rewarded through the block reward this is taking note that if PoW is not used, a substantial amount of rewards are able to be distributed while at the same time distributing fees coming into the network from premiums.
You have to take into account that it does not have to be 1:1 the whole time if some people are storing extra files they will have no problem with it at all if they are rewarded accordingly this can be with energy/gas and/or financially. It going off ratio does not mean that it is a attack. These numbers would have to be played with and the only way doing so would be actually getting a test net up and running