Ethical Problems Brought by Academic Paywalls
This seems contrived, as normally you can at least read the abstract to find out if you need to read the paper.  

If you do need to read the paper, you have valid ways to do it, such as interlibrary loan, without paying $40.  It's not free, but not a huge expense.  You can opt to not read the paper, in which case you'd be doing your professional practice a misdeed.  

Lastly, there is ALWAYS similar research by other people, who have not seen or heard of your paper.  It is NOT plagiarism to independently come up with something on your own.  There would normally be no charge of plagiarism, and you are wasting your effort serving as a patent troll.

If you are a publisher charging exorbitantly for access, you would build up your reputation, hire editors of stature, maintain a staff that keeps papers moving through the business, maintain a list of good referees, and do many other things to try to add value to your service and repository.  If you build up your reputation, institutions would subscribe.  If you can't establish a reputation, you'd go broke.


  Should the poor guy suffer a plagiarism charge just because I kept my
  research behind a paywall that he/she cannot reach?


The most likely scenario is that the person will not face a charge because no plagiarism occurred, as your paper has not been read.  Another likely scenario is that the paper were acquired through legal and nonexorbitant or free means like interlibrary loan, prepublication sources as required by NIH under the Public Access Policy, etc.  

There are sound reasons to advocate for open access, but the fear of forcing plagiarism probably isn't in the hit parade.

Lastly, I encourage you to find the papers you need by legal means, so no, I don't feel acquiring the paper for free through extra-legal means is the way to go.