In past threads, we also considered the idea of using from __future__ import dedent_string_literals instead of the d-prefix.
So if future import had been chosen there, I would have designed the rules to prioritize maximum compatibility with all multiline string literals.
However, people chose the d-prefix over future import. We do not need to replace all """...""" with d-strings. I designed the rules with priority given to usability in contexts such as textwrap.dedent("""...""").
Therefore, I prioritized eliminating the inconvenience that a \ is almost always required immediately after textwrap.dedent(""", over the inconvenience of needing to add a newline when rewriting """ as d""".
When making technical decisions, many objective facts are taken into consideration, but the final choice of priorities is always subjective. My subjective judgment may not align with yours; however, since I am the author of this PEP, it is ultimately written based on my own subjective decisions.
The presentation of new facts that support your position is welcome, but let us refrain from continuing subjective and abstract discussions about whether the initial newline should be preserved or removed.