The trade-off you seem to prefer is to make the "plain text is universally readable" idea from the core Unicode definition, not applicable to BiDi text.
Your idea would simply outlaw being able to view text with a reader-defined stylesheet imposed on it. Such a stylesheet should be perfectly able to impose a paragraph direction.
Just as you might make sure that your application gives you a choice of using a stylesheet that "imposes" default paragraph direction.
Just because your choice makes the most sense to you in the scenarios that you imagine, doesn't mean it should somehow be the only choice.
There are cases where such flexibility is much less motivated and, if allowed, potentially more harmful - therefore, you will find little in the way of optional (higher-level protocol) stuff for the basic encoding or even normalization.
A./
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