Just because I have to:
I sort of made this change...I sold my 28-300L last year, and with the proceeds was able to buy a 70-300L ($1,075) AND an old but clean 35-350L ($650).
Best of both worlds, a great telephoto zoom when you need 300mm, and a very capable "all in one" lens when you need it. I couldn't detect a difference in IQ between the 35-350L and 28-300L at similar focal lengths, and was willing to trade IS and the 28-35mm range for being able to buy a 70-300L. The superzoom was mainly used outdoors or if inside, with a flash, anyways.