This. And honestly, 10k sales is the bare minimum. Even if you’re a solo dev with no team and you handle everything yourself (programming, sound, music, art, marketing) to keep costs down, you’re still looking at around 6–12 months of work.

Most indie games don’t sell for more than $10 USD, but let’s be generous and say you manage to convince your audience to pay $20.

  Total: 200,000 USD
  After Steam Cut: 140,000 USD

And now you need to get lightning to strike every year to maintain your annual income so you can retire before you're Methuselah.

Could you work on the game part-time while holding down a full-time job? Sure, but you've got to have some iron stamina to want to sit in front of a computer for another 4 hours after a full day of work. Furthermore, not being able to focus on the game means dev might take significantly longer.