To put 770 at the KIRO site would require a 3rd tower. The KIRO towers (only 2) are taller and spacing might require more land. 770 was originally diplexed on the 1090 towers and works well. They would have to rebuild the diplexor system at KIRO add the tower if possible, move the phasor, transmitters. Redo the air handling system. Plus a lot of engineering (again two 50KW 60Khz apart, two towers that are taller) and new transmission lines, digging, and FCC filings. And the spacing of two 50KW 60Khz apart works better on separate tower sites. Lots of money would need to spent in the short term.
When Bonneville bought KING 1090 from KING broadcasting they first executed the frequency swap between 1090 and 770, I don't think moving 770 to the 710 site was ever considered beyond conversation wonder talk. Bonneville only owned both station for about 2 years before selling to Entercom in 1997. I think any owner would just keep things as they are in case one station or the other sells. Entercom spent very little money while they had the two stations. But Bonneville bought them both back in 2008. There is no real good reason to move 770 to the 710 site.
I don't think CBS wants to own the site. The CBS rent basically pays the power bill for KIRO, KTTH and probably some of the program and remote control circuits. I feel CBS has the better rental deal since the site has a real flushing toilet!
Back in 1974 when my dad took the week end engineering position at KIRO, and I would walk out to the tower bases with him. I wondered if in my lifetime, would I see the end of 710am. As in it's obsolete and shut it down. Fast forward to 2015 and now I'm responsible for it's care. I may very well see that sunset. They might invest in a new solid state 50kw transmitter, but not moving 770 to the 710 site.