Symbian wasn't really designed from scratch though. It has its roots in the nineteen eighties. The 32 bit version of Psion's Epoch is what eventually became Symbian and it wasn't until other consortium members gave up that Symbian was taken over by Nokia.
I worked at Nokia Research and Nokia Maps between 2005 and 2012. I witnessed the arrival of the Iphone and Android from the inside. There was a lot of disagreement internally on what the right thing to do was. And part of the company was actually very clued in and trying to do smart things. But Nokia did a lot of things wrong and most of that was just not getting that it was a software company. It had all the wrong reflexes.
The problem with Symbian from day 1 was that at the time it launched, it was already getting clear that embedded Linux was going to be a thing and it was a bit lacking in features relative to that. Between the late nineties and 2005, there were quite many attempts to use Linux on mobile and embedded devices. By the early 2000s it was widely used on things like routers and other devices. And there had been some attempts at building pdas and phones with it. Google bought Android (the company) in 2005; and they launched phones around 2008. 2005 was the year I joined Nokia. Rumors about Apple working on a device were already getting quite concrete around that time (i.e. Nokia execs would have had good intel about what was coming) and over the course of the next few years it became clear that it was going to happen.
Nokia was focusing on flip phones instead. They were really worried about the Motorola Razr. And the Blackberry. They completely missed the point of desktop operating systems being repurposed for mobile. They thought mobile was special and that they owned it. They stubbornly ignored all the internal signals (there were many) that that was wrong. Nokia even launched a Linux based device in 2006 the N770. It was not a phone and that was not an accident. Operators would object. This could not be. So, they crippled it.
Apple and Google both proved them wrong. The Symbian strategy was dead as a doornail before they even launched the first devices (around 2003/2004). Apple proved that operators were weak and could not ignore popular consumer demand. And of course IOS was a specialized version of OSX for mobile. And Google ended up benefiting a lot from Nokia's Linux work. The kernel was essentially the same for Maemo and Android. Google even bought N800s to dual boot them into Android before they had the first Nexus phone ready. The point here is that Nokia had a shipping touch screen based Linux device in 2006. Years before Google or Apple had phones on the market. The only reasons it didn't have a sim card were political. It could have been a phone, easily. The N800 continued the tradition. It even had a webcam and skype. But no sim card. Only the N900 fixed that. But that was way too late and they positioned it as a developer toy. Innovation around this topic was stifled. It was the obvious move. But the Symbian crowd successfully blocked and frustrated that.
By 2008 it was scrambling to undo a lot of really bad decision making around Symbian in the years before that. Which included actually cancelling S90, which is a touch screen version of Symbian that never saw the light of day. That happened around 2005. S60 3.x was the current version when Apple announced the iphone and had no touchscreen support at all. Nokia had to rapidly create a new version with touch screen support. It was a rush job and the first versions completely destroyed Nokia's reputation because it was unstable and unusable (both). The launch device for this was a complete flop. And it took until the early 2000s to stabilize it. And by then the phone reviews were brutal for any Nokia device. Apple was running circles around them. And Android was starting to actually eat into non Symbian (S30, S40) Nokia revenue. Which actually were most of the market. Nokia's entire OS strategy was failing at that point.
It's feature phone market started imploding and this was until then the money maker. They sold hundreds of millions of those. And then we got the whole drama with pissing in your pants to stay warm (Ansi Vanjoki, one of the VPs) and then Stephen Elop getting hired and chucking him (and Symbian, Linux, and all the rest) out. And then Windows Phone of course didn't make it and the MS acquisition happened and MS unceremoniously pulled the plug in 2014. Too little too late.