If you buy now and canon releases something else down the line, you're still ahead.
I bought my 16-35 II for $1250 back in 2008. I sold it last year for $1218 on ebay. It's like I had a free 6 year rental and have thousands of images to show for it.
If your gear is clean and looks pristine, L lenses hold their value exceptionally well. Take the pictures you want now and sell it for little to no loss, using those funds to buy the next new thing that comes along.
Thats very true. I usually wait until lenses hit a low point on price when I buy. I have done very well selling them later. Canon L lenses tend to hold their value very very well.