I don’t think you’re being entirely unreasonable, I’m just not sure it’s possible.
It’s not happening by design, that would imply intent or bias on what the site shows on all. It’s happening naturally, all literary shows everything to everybody. Claiming it’s by design is like saying it’s by design when your entire office votes “no” to you banging on drums during the workday. It might be they just don’t like that idea.
Remember that the situation, the configuration you’re referring to, only existed on Reddit because those subs banned people left and right, and people blocked them. In other words, they were and are isolated. r/conservative would ban people for any dissenting comment, and conservative subs got delisted from r/all for a variety of reasons by the Reddit admins.
The only way to achieve what you’re describing would be to do the Reddit thing and pre-select magazines for the “all” tab, rather then them truly being all. Even if subs were the main page, people will still flip over to all regularly, just like they flip over to new.
You may just have to be content knowing that people that subscribe to /m/conservative go there intentionally despite the posts falling down on the all tab. Maybe there will be more people here that agree with you and the situation will change.