Here's my pitch. I think at best Shadow Complex was a bit of a wake up call for indie devs in the sense that it showed them that they were "allowed" to make a game like Metroid.
I know that might seem silly but the human experience is silly
Obviously people have been making games like this forever but sometimes you need a flash point for it to hit home with people. Think of every big thing in games and you'll generally find the starting point, the thing that spawned copycats, didn't invent it. They just sold the most copies.
Shadow Complex did an incredibly 1:1 adaptation of the Metroidvania formula, where as classically games in that loose genre (Faxanadu, Monster World, Blaster Master, etc) tended to be a little more bespoke, that got tons of attemtion, made lots of money and received no push back from Nintendo or Konami.
I can see the case being made that overnight a hundred indie devs who had been saying for years, "Why don't they make another 2D Metroid/Castlevania on console" just going, ok, I'll do it myself too then.