How to add more quality to my nikkor 28-70 2.8 ED lens which is 10 years old?

Its very costly lens near 1600 dollar when bought. But nw the picture quality is like very bad. That means a fade out in colour and more fog in the images. The lens was already cleaned 3 times by professionals. But nw the picture quality is worst than taking in a 18-105 ED lens. So i'm looking forward for any thread that relevant.

Which professionals did it and what did they do? Did they dismantle the lenses at all or just clean the outer elements?

Check the link and see if anything matches.

Look through the lens. You should see that everything is clear. If not, it's either a fungal bloom or a separation of coating. Your lens is old. Cleaning can only do so much. It really is a pity. If you want your equipment to last, invest on a dry box.

Your lens, with the right amount of light will do great for soft focus shots. Keep it but keep it away from your newer and cleaner gear.