Strong Town's argument is that some of these suburbs will be successful in encouraging mixed-use development and in doing so will expand their tax bases. But most of the others will fail and decline.

ST are quite explicit that they mostly don't think that retrofitting suburbs should be a focus, because they believe that most of these will go bankrupt anyway, given a sufficient amount of time (because of too much infrastructure liability compared to the tax base), and indeed this is starting to happen all across the US. Hence their focus on areas with a solid core already, that should then be strengthened and encouraged to expand organically from there.

But that's just it. If someone likes suburban living, and your family home is in a suburb, how do you think they'd feel about something like "suburbs are probably all going to fail anyway, so let's do a bunch of things that will definitely and quickly make them fail, and then await the glorious day when your house gets torn down and replaced by apartments, and your neighborhood becomes a replica of the inner city"?

I don't understand your reasoning.

The only thing ST says here is "let existing suburbs be what they are", which sounds like what you want as well.

Even if they're not directly changing the suburbs with this proposal, the changes they want made to the city will make life worse for people who go there from the suburbs.