Oh man, you should have seen the mobile sphere before Android...
iOS didn't move the needle on open source, open bootloaders or anything gp mentioned. All iOS did was shift the balance of power from telecom companies to Apple.
Android was far more open than all that came before it: the source code was available,it used open source & contributed back, and brought a very "PC" mindset[1] to an industry that had always considered handsets to be appliances (save a few Nokia models in the Communicator line).
1. "Intents" that could be handled by any app of the users choosing was very much like "Open with..." on PCs. Even disregarding the open source stuff, Android was very much an open system just for that reason.
Android can be more open but let’s not pretend that a lot of devices have locked bootloaders, too, and has never lived up to that “PC” model due to things like proprietary drivers and apps refusing to run on unlocked devices. The future is very much unsettled here and we need something like legislation to reverse that trend.
Not all Android handsets have unlockable bootloaders - but all handsets with unlockable bootloaders run Android.
However, my main point was that outside of niche devices[1] that are rounding errors for smartphone consumer market share: the vast majority of devices sold with unlockable bootloaders already run Android: I'd guestimate > 95%. Looking at the list of devices supported by LineageOS (after enabling "Legacy Devices") is awe-inspiring. In the past, Samsung, Sony and Motorola had a solid lineup of unlockable devices.
1. I say this as someone who almost succumbed to the temptation of buying the Pine64 phone with the intention of contributing code
Android would not even have launched without the iPhone breaking down those doors, you can hear it first hand from the Handspring/PalmOS/WebOS folks: https://youtu.be/b9_Vh9h3Ohw?t=1609
Around 2007 we tried building an app that scanned barcodes in a store to show you comparison from online stores. Symbian's APIs for cameras were so broken, that we couldn't get a decent enough resolution of photos. Java had permission/ux issues that made the app unfeasible. It was literally impossible to build an app like this back then, even though phones had good enough cameras already.
Not to mention the fact that you needed a whole studio to build and test even a simple app like this - so many variants of OSes, and phones, and every single one had it's own quirks.
And to add an insult to the injury - we had to pirate the SDK, because the official links to SDKs didn't work for a few days, and nobody seemed to notice.
So yeah, in theory the users could download apps to the devices. But in practice it was easier for devs to hack into iPhone 1.0 and build apps there - even before the official app store launch, than it was ever to build something for Symbian and the others.
What probably did happen, a bit, was that ISVs that made software for other markets, may have ported their software to PDAs, and sold that. (E.g. "WinZip for Windows CE", things like that.) But that doesn't mean that these companies were surviving off of this. These were effectively "halo products" (products that don't sell many units, which are made instead to increase the brand perception of key influencers who happen to have such niche interests, and thus influence their [influential] opinion of the rest of the product range through the halo effect.)
Because of that, even before the official launch of an iOS SDK, there were more apps for iPhone (because people hacked it), than there ever were for Nokia.
For WindowsCE, sure there were apps, but phones didn't run Windows really then. Only PDAs.
Wait, also a whole freaking browser : Opera mini - don't say that isn't complex !
Yes. And it was marvelous.