[bigiain]

1) The block rewards are going to halve twice during those 7 years

2) the hardware is going to become obsolete well b4fore 7 years (although I wonder how the semiconductor shortage is holding back custom mining hardware design and manufacturing? Maybe in 2029 people will still be using today's hardware? I seriously doubt that.)

3) I'm reasonably sure arbitrage pushes the cost of mining asymptotically close to the price of electricity to run it, so the margins are almost all whatever temporary, fraudulent, or criminal steps you can take to reduce your cost (or outright steal) the electricity.

[FabHK]

In 3) s/arbitrage/competition, and apparently it is not only the electricity, but also the capital cost of the mining gear that you have to take into account.