Maybe I need to ask a more specific question.
Sennheiser (and Chinese cloners) are clearly able to make microphones with similar housing to the MD409. What is preventing them from creating a capsule similar to the MD409? In various tests, while the e906 is closer, all the modern takes on it have a presence peak and sound/measure differently. They used to be pretty consistent, even when they launched the "Blackfire" range.
The same question could be asked about Beyerdynamic's M380, or Shure's SM5B.