Tho Minecraft was something like the "proto Fortnite".

All the casual gamers I know spend their time on Minecraft, at least until Fortnite came around, then they've pretty much all transitioned to Fortnite, with Minecraft now being the "if Fortnite doesn't work" replacement.

Is Minecraft's usage decreasing for real? And hard numbers?

Minecraft doesn't have a single huge social/multiplayer server so when players reach their teens they tend to migrate towards more social games with lots of players, Fortnite being the prime example, and Roblox being another (cringe). The current dominant Minecraft server, Hypixel, is developing it's own 'voxel-based' game called Hytale. It will be interesting to see how that goes once it is released.

All video games have a shelf life. The experience gets stale, the hype dies down and people move on to something else.

My 12yo has grown out of Minecraft, but 6yo shows signs of growing interest. This week, she discovered redstone and pressure plates and went automating every door she can find in old builds she normally doesn't care much.

Minecraft will have endless supply of users in the future, and when they're done with it, there will always be a next step waiting. That's why Netflix will always have trouble figthing with those.

With something else being another game. People don't switch interactive entertainment for non-interactive one.

Are there studies showing that? Switching seems perfectly natural. If a good game comes out I'll play that. If a good TV show comes out I'll watch that. And since my free time is limited, if I pick one it means I'm not doing the other.

Same for me; though I prefer to play an old game than watch new TV quite often.

The TV show end, multiplayer games don't.

CSGO: see a few people in every match (so perhaps 10%) that have the 10year badge.

Some games have lasting replayability.