I still don't see the logic behind this, sorry. Whitehall can only turn a few (7?) tph, and lower manhattan riders already have access via the line, which will always be faster than taking the QBL local there. Furthermore, if you end the at Astoria, you're eliminating direct access to a yard, complicating operations and clogging the West End line with OOS trains. Extending the doesn't involve any yard swapping, and provides the Rockaways with more frequent service. The only drawback I see with the extension is that one doesn't have easy access to the 7th avenue lines. I'd say because the area is straddled with the 6th and 8th avenue lines, it's OK.