This does precisely nothing to solve the ambiguity issue when a final line lacks a newline. The representation of that newline isn't relevant to the problem.
The point is that having a sequence of two delimiters to signal the end of the logical line allows you to have single instances of either delimiter included within the text. This allows visual line breaks to be included within the same line as understood by the regex parser.
Despite the downvotes your comment received, I think you have a good point. There are two uses for a newline, first to signal the end of a line, for example when sending text over a serial connection, and second to separate two lines, for example in a text file.
To indicate that a serially received line is complete, the interpretation as a terminator makes perfect sense - abcd\n is a complete line, abc is a still incomplete line. In a text file the interpretation as a separator might be preferable because that gets rid of the issue of the last line not having a newline - a\nb\nc are three lines separated by two newlines, a\nb\nc\n are four lines separated by three newlines and the last line is empty.
But then it might also be useful to have a terminator in a text file to be able to detect an incompletely written line. So using two characters, one for each purpose, could solve the problem. \r means the line is complete, \n means it follows a next line. abc is an incomplete line, abcd\r is a complete line and no line follows, abcd\r\n is a complete line and a second incomplete line follows which is currently empty. abcd\r\n\r are two complete lines, the second one empty. abcd\r\nefg is a complete line followed by an incomplete line. abcd\r\nefg\r are two complete lines. You could even have two incomplete lines abc\nefg.
But I think Windows always uses \r\n because this is how you get to a newline on a typewriter or really old printer, you return the carriage and feed the paper one line. I do not think that they had the idea of differentiating between terminator and separator, otherwise you could have only \r and maybe even only \n sometimes. But in principle this could work quite nicely, I guess. You could start a line with \n and end it with \r, this would give you \r\n between lines and \r after the final line. Or nothing if the final line is incomplete or \r\n if the final line is incomplete and currently empty. The odd thing would be a newline as the very first character, maybe one could suppress that. This would also be compatible with Windows and nix, it would just consider all nix lines incomplete. Only abc\rdef\r would not really make sense, two complete lines but the second one is not a new line.
If I ever get to write a new operating system, I will inflict this on humanity.
I mean, it was what everyone had agreed upon previously. Microsoft was the only party to follow through. For all the guff they get for not following standards, it was the one standard they did.
You don't have to love a company to acknowledge they did something right.
