Sorry, your memory is wrong. I've owned Nokia 5800 back then (2009 or 2010, don't remember exactly), and had a chance to compare it side to side with HTC G1 and iPhone 3G. It felt dated. Many apps were not designed with touchscreen in mind, the standard UI was "list with two buttons on the bottom", grandfathered from phones with keyboards. Resistive touchscreen didn't help either.
Then, the mobile hardware was making quantum leaps each year. I don't remember the exact numbers, but it felt every next iPhone or Android phone was twice as fast as the previous generation. And Symbian was stuck on a basically single hardware platform, with a single supported chip and a single weird display resolution (640x360).
Symbian^3 was a step in the right direction (at least they managed to rebuild the kernel for the different chip!), but the legacy was too heavy, and the competitors were moving much faster than Nokia. Like, the last Symbian flagship (Nokia 808) still had that 640x360 resolution on 4 inches. At that time, iPhone had HiDPI displays for two years!