I guess. And I guess Roger at Lens Rentals got a good copy - http://www.lensrentals.com/rent/canon/lenses/telephoto/canon-70-300mm-f4-5.6l-isI trust you realize that you linked the 70-300L? While Roger does say that the 70-300 is 'a very good consumer zoom' (consumer being a key point), he also says the 70-300L 'puts it to shame,' optically and in other ways.
Lots of comments from 'real users,' including many in the POTN thread you linked, about the lens being soft at the long end, advice to be sure and stop down at the long end, and even so, statements like '300mm @ f/9 still could not produce crisp images.' The two copies of the 70-300 non-L that I tried were also noticeably soft from 200mm onward.
I'm glad you're happy with your copy of the 70-300, as I am sure many people are. There are also people who are happy with one of the 75-300 lenses. I notice you stated that the 70-300 isn't worth $600 (it's currently $650, BTW). While paying less than half of current retail doesn't affect the optics of a lens, it can affect one's perception of that lens' performance, and it certainly affects the perceived value. Personally, if budget was a limiting factor, I'd recommend the 55-250 STM for a crop body, and I'd suggest FF users beg or borrow the $60 to cover the difference to the 70-200mm f/4L (a difference which drops to $22 if you buy the ET-65B hood, not included with the 70-300).
I personally agree that the 70-300mm IS USM is not worth $649, though I paid $380 for a brand new copy and therefore I am satisfied with its performance. It *is* soft at 300mm and the 70-300mm L does destroy it. However, for under $400 prior to the release of the STM version the non-L was by far built the best and looked the best. I believe the STM version will likely outclass it optically but still be an inferior build quality.
Regarding the 70-300mm f4-5.6L vs 70-200 f4L, I believe they are for two different audiences yet in many ways I see the former as more useful than the latter.
- Slower and a bit heavier BUT...
+extra 100mm of SHARP reach.
+easier to fit in the lens bag, as generally width is easier to accommodate than long length
+A nice complimentary lens to the 70-200 f/2.8 L, rather than one you'd want to replace due to extra reach and small size.
+A far higher IQ "travel lens" alternative to the 70-300 DO IS
+Faster @ constant F4 and a bit lighter BUT...
-100mm less reach. Try to add a TC and you have even longer length and slower lens than the 70-300L.
-longer length might make it a difficult fit in some bags
-If you already have the 70-200 F/2.8L, this lens is rather redundant compared to the 70-300L
If you don't have nor plan to get the 70-200 F2.8L ever than the 70-200 f4l is a better buy. But if you do have or plan to get the 70-200 F2.8L I can see how the more compact length and greater reach of the 70-300mm L may make it a nice lens to keep in a addition to the 70-200 F/2.8... But I think one would be less enticed to keep both the 70-200 f/2.8 and the 70-200 f/4 due to the exact same range despite weight differences.
TO address the OP...:
Yes, I believe the NON-L 70-300mm is due for an update. While it is better optically and build wise than the old 55-250, it is unlikely to retain the optical edge over the 55-250mm STM and the autofocus will definitely be worse. Being that this lens has been available with rebates @ under $400 several times in the past few months, I believe Canon is going to retire it. It may not be replaced, though, as someone else mentioned the 70-300L may have actually been the replacement. For those that want something smaller and lighter than the 70-300L there are already options with the EF 70-300 DO IS and the EF-S 50-250 STM.