I originally did not like the idea of a push-pull design, but have changed my mind after using my 400L for a few years.
Some of the advantages that this design made available when it was designed are:
Short Storage length, shorter than the 70-200mmL
Close focusing, for filling a frame with small birds which often let you get close.
Is has worked for me down to 1/20 sec, far better than the IS on my 400mm f/5.6.
Newer lens technology now allows for shorter lens designs, better IS, all with close focusing. I'd certainly welcome a new model that had those attributes, push pull or not. I'd prefer the smallest one, whichever it is.
My 400mm f/5.6 is too long to easily store, and won't focus closely, plus a lack of IS can be a issue for stills where light in not its best. Having to use a high shutter speed requires high ISO's unless I have bright light.