I don’t want to necessarily argue about merit or quality, but I have seen that custom software can be accessible to a general audience.
But how does that contradicts the idea of extendable software? If anything, extendable software will better do what the user wants it to do.
It's just right now we think of extending the software as if it's a complex operation. It looks to me like the article envisions a future, where extending your software is as simple as, say, creating a new google doc or opening a link.
It is doubtable, however, that such level of ease of extendibility can be achieved at all. There might be enough essential complexity, like answering lots of questions, which, I agree, most users won't be doing.
Think about cars for comparison. Some people like tinkering with them and tuning them. However, most people look for the features they need or desire and buy the car that satisfies their requirements and budget best.
Sometimes extensibility is part of the requirements to solve a problem or fulfill a desire but normally it isn't.
However, I can see another counter-argument to my claim: not every consumer has specific requests for the product they're buying! Sometimes people are just not sure what they want. In that case, yes, I can agree that customization doesn't help. Moreover, having a limited set of pre-defined products helps to educate a consumer on what their options even are.
So, if a large part of consumers are the ones that don't know exactly what they want, then yes, you're right. I guess the distribution of “know-what-they-want” vs. “don't” depends on the product, although I think generally it tends to be that most people usually don't know.
Anyway, thanks for the good argument! Thinking about other products and industries, as cars, really helped, I missed that.