It's not best practise to make loads of folders and sub-folders. Consider other ways of structuring your documents. You mention '10 different project libraries' - what's wrong with this structure? It's easier to customise the security, since perhaps different project groups / departments need different permissions for each project?
Using multiple document libraries allows you to:
create columns (fields which might apply to one project, but might be meaningless to another project).
customise security; think 'who needs contribute permissions (read/write), who needs 'read only'?
Check out an answer I gave to another question, there is a link to a blog in the my answer which I think you'd benefit from reading.
In response to your comment; if you want to move a lot of these documents in to a new Document Library create a new column called
Archive - with the options of Yes/No. Use this new column to tag documents with a
Yes if they are old documents. Then configure a view to either include or exclude any files where the
Archive column is equal to Yes.