There are some pretty significant reasons for the phenomenon that you are observing, though it is definitely contentious how stagnant or uninnovative it is as counter-examples are aplenty, but I think that the reasons are a little more complicated than simply declaring that indie game designs have become stagnant or that its somehow their fault.
TL;DR: the market landscape has changed and people are still building and expanding on that initial wave of game design innovation.
A lot of the indie innovation happened around the time of 4e D&D (2008-2012) and its dropped support (2012-2014)* and shortly after the release of 5e D&D. No joke. I have even seen several indie games designers saying that 4e D&D did more for indie TTRPGs than 5e D&D ever did. It was also around this time that the OSR movement was really kicking into high gear and formulating its game principles. But after a time, B/X basically became the gold standard for OSR (and games like OSE and SWN/WWN have become its standard-bearers), though this is not to discount all the non-B/X-based OSR out there.
* Gumshoe (2007), Fiasco (2009), Apocalypse World (2010), Cortex Plus (2010), Dragon Age RPG/AGE System by Green Ronin (2010), The One Ring Roleplaying Game (2011), Star Wars: Edge of the Empire/Genesys System (2012), Hillfolk (2012), 13th Age (2013), Fate Core (2013), Numenera/Cypher (2013), Torchbearer (2013), Mutant Chronicles 3e/2d20 System by Modiphius Games (2013), Mutant Year Zero by Fria Ligan (2014), Tiny Dungeon 1e (2014), Symbaroum (2014), Shadow of the Demon Lord (2015)
What changed? 5e D&D happened. This is not to say that innovation died. It didn't. However, 5e D&D definitely changed (and dominated) the TTRPG landscape. The people and money are there. There was a better chance to break out with your game design in that 4e D&D era.
I think that we are also in a phase where a game was released and it has taken about 5-10 years for people to understand and expand the system. This is why we are seeing games like Ironsworn (2018) and Blades in the Dark (2017), which take PbtA into a different direction. And likewise, we are still arguably in the early phases of the Forged in the Dark, and we will see how these designers push that design into new directions as well. And I do believe
@Fenris-77 in good faith when he says that Swords of the Serpentine innovates on Gumshoe.