"Ignoring the bad markup isn't a complete solution. Janitors still are likely to be asked to fix it."

I wasn't offering it as a complete solution. I don't feel there is a complete solution and, as I've said, don't believe it would be worth the work for essentially a handful of weekly problematic posts. As for Janitors, that's part of their purpose:

From What do Janitors do?:

What about crappily formatted postings?

...On the one hand, it could be left to the author to fix, and a /msg to the author is therefore the best course of action.

On the other hand, a janitor may feel that cleaning up a poorly formatted node, as a service to those who have to read such a node, and as a way of illustrating how nodes ought to be formatted, is reasonable.

And if the node's author is Anonymous Monk, it can't be fixed by the author, so if it's going to be fixed at all, it's got to be done by janitors. ...

 "As to posting plain text, I've never encountered difficulty including some... as in the case of the first (untagged) para in this node, nor in the case of a single sentence of narrative reply. (alternating paras with and without tags maintains readable rendering)"

What I posted was all plain text and got the following under the Title form field:

Use:  <p> text here (a paragraph) </p> 
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":
[download]

That was trying to post from my phone in all plain text. What you posted above is readable so I don't see a problem. Unless it brakes the page, I don't think we want to start parsing for totally valid html. That's not even a goal covered in the Janitor's doc and, as it suggests there, a private /msg to the OP can help remedy the situation. If it is a recurring problem from a specific poster suggest they change their setting to show markup errors. This is functionality which already exists but doesn't block a post from being submitted.