The backup needs to be deleted the moment you don't own the media the original is on anymore.
You own the license to use the software, the software itself is protected by all kinds of copyright laws and you don't own it.
A right you don't have for instance is to adjust the original you bought and resell it, with exception of original updates provided by the copyright owner.
It's simular to books. You can resell the book, but you have no right to the text in it.
You only own the physical product, not the content.
The fact you can do more with it doesn't mean you have the right to do so.
I can remove a brick from the side walk and throw it through a window.
The fact I can do it doesn't automatically give me the right to do it.
It just so happens in my country(Canada) software is, for the purposes of copyright laws, a book.
In most EU countries music, movies, books and software aren't equal in copyrights.
Especially software often differs.
For example, here in The Netherlands we are allowed to make an unlimited amount of copies from music, movies and books for every person within your household (not family!).
With software we're only allowed to make 1 backup for safety reasons.
I know there are countries where you're not even allowed to make a backup.
Circumventing copyright restrictions on all is very shady.
We're allowed to crack it without help or tools from others and for learning, research or security reasons only.
However as far as books, music and movies it's legal to download them without ever buying a copy.
In some other EU countries they also have different treatments for different kinds of media.
In others they're all treated equal.
Renting copyrighted material to others is prohibited here without permission of copyright holders.
Obviously you're only allowed to sell the original, not the copies you made.
I think I was clear in that in an earlier reply.
In some countries you are forced to destroy the copies you made.
In mine that's the case with software.
There recently was a case about leasing unused software licenses to a 3rd party either in The Netherlands or in Europe.
The court ruled that the company who leased it to the 3rd party wasn't allowed to do so without permission of the company they got the software from.
If I buy a product here with DRM in it which causes it to not work as expected, I'm not allowed to circumvent DRM.
However I am entitled to getting my money back.
We had a few cases here with DRM protected CDs which didn't function in car stereos.
Philips and Sony (inventors and rightholders of CDs) even addressed record labels about it as they often incorperated the official CDDA logo which is an ISO standard.
Only exception to this is when it's clearly noted on the package, which includes more than just mentioning the presence of DRM.
Unfortunately a lot of publishers and retailers don't keep to these laws.