[buboard]

it's not the same file - just the same, final text proof. It will be different from the final formatting in the journal.

I dont think authors have incentive to abuse the system. Just upload the final proof of your manuscript to arxiv, click "final version" , and this lets people know that this is the same article as in the journal.

DOIs are ubiquitous and they would serve the purpose of redirecting to the free pdfs rather then the journal site. This can be applied to existing articles retroactively. Plus, many bibliography styles include the DOI which makes the reference easier to use

[Symbiote]

DOIs are controlled by the journal publisher, so I don't see why they would be willing to change their target.

[buboard]

yeah good point. Maybe doi.org could resolve them differently? In any case it s the reference identifier that could connect the two documents