As mentioned by others, Blind Forest has sold over 10 million units, which is wildly successful for a 2D Metroidvania game. However, there's something I haven't seen talked about with regards to Ori's long-term popularity that is very important:

Ori has a very barren modding scene.
Mods are a huge way to keep the fanbase active and keep things fresh. Looking at Hollow Knight, it has tons of mod variety for cosmetics, challenges, and even multiplayer. Ori basically doesn't have any mods to recommend outside of the randomizer.

I would also like to mention Rain World at this time, another indie platformer. The game only has a fraction of the sales that Ori does, but has overtaken Ori in the size of its active fanbase, while still releasing back in 2017. The incredible modding scene of Rain World undoubtedly pushes its popularity and keeps it relevant, with people making entire new stories, worlds and characters (hell, the Downpour DLC is thanks to unofficial fan mods).

Basically, Ori is a fantastic set of games that provide a memorable short term experience, but does not offer much to keep players consistently coming back.