There were major components people refused to even touch (like messaging) because the code was basically unmaintainable. An aggressive refactor of the entire OS was needed but they never really got comfortable with that. Symbian was originally essentially an operating system targeted at a calculator that outgrew itself several times over.
The first modern slab phone I ever saw wasn’t an iPhone… it was a Nokia prototype in like 2005. If they had a real operating system in place maybe Nokia is the world’s most valuable company today.