I believe that the biggest problem with Self and later Squeak was never releasing a useful (killer) app that could demonstrate the advantages of the system. If you nowadays go to Squeak and open a standard image, there is nothing that you can say: this is an useful app that I would like to use. Even Plan9, which is by comparison a very esoteric OS, has managed to create useful apps such as Acme.
I would hardly fully credit Plan 9 for ACME, given that it just made Oberon text editing famous, and poorly given that it lacks document object model from Oberon.
In any case, the main issue of these platforms is a bit like explaining monads and other complicated stuff.
To really get the point of how those systems work, a person needs to invest time into the system until it clicks, and no matter how much we try to put the experience into words, there is only so much the audience is able to grasp, assuming it actually cares to pay attention to.
Killer apps, when it comes to languages, are feedback loops, but you need a statistically significant number of people using those languages to make it more likely that those killer apps get made. To me, this really just sounds like statistically no one used Self so no one (or not many, to be kind) made useful apps in Self.
imo, the Self/Smalltalk vision was just hard to sell to people in the 80s/90s since it was so ahead of its time, with no easy bridge back to the world people were building their existing software in. It might even be hard to sell it today. People built their apps with an existing toolchain and a language they already knew. To adopt the Self/Smalltalk vision largely meant ditching all that to replace your language, your OS, and your toolchain at the same time, for a bet on the claim that a dynamic language was somehow fast enough.
Instead, we piecemeal evolved into a worse world where Javascript apps run in a worse OS (the browser) and Python apps run in a lower level OS that needs to transport itself in a worse image model (Docker). But, it was easier to adopt from what came before. I can't say I'm happy about it, but at least the vision of Self and Smalltalk caught on.
