I wouldn't give up on the lens....you just need to pick your circumstances and lighting more carefully. For ages, until I got my 50-500 Bigma, I used a Quantaray 70-300 and had many shots like you posted here. I also had many more that I was well pleased with. I learned to avoid overly bright situations, reduce exposure, and concentrate on getting as close as possible for sharpness. Purple fringing can be quickly removed in the modern programs available now.....so it is little problem, although the 70-300 is prone to PF with a bright background.
Here is one I like from the K10D and Quantaray 70-300....not near perfect, but still makes a nice print.
Keep experimenting, you will get more good ones than you think is possible with a little work!
Same lens.....70-300
Anything you find here shot at 300mm was with this lens...some decent ones ...some so-so....
LOCAL BIRDS Photo Gallery by James Williams at pbase.com
Best Regards! & Good Luck!
Rupert