Not just a fear, but a reasonable conclusion. Obviously sandboxing won't be effective for third party software unless 100% of all third party software is enforced to use sandboxing and won't run without it. It's pretty much an inevitable conclusion that that is coming because the system doesn't work unless you go all or nothing.
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Your opinion is valid. There is one way to disable it as these rules still only apply to apps sold via the MAS.
Anybody can release software outside of it and just forget about Apple's rules.
I think the fear is that Apple will decide that the MacAppStore is the only way to install software on OSX.
Not just a fear, but a reasonable conclusion. Obviously sandboxing won't be effective for third party software unless 100% of all third party software is enforced to use sandboxing and won't run without it. It's pretty much an inevitable conclusion that that is coming because the system doesn't work unless you go all or nothing.