Even then you'll be placing huge orders. Towards the end you'll have over 1m. Even if you split that across 10 coins you'll be placing 100k orders, that's a big wall in almost all markets except for the few biggest such as bitcoin and ethereum.
I'm only speaking from experience, I never got close to $1m but even just going from about $100 to $10k doing the same but aiming for 3% I found that in the early days I could easily get closer to 10% but later it was a struggle for 3% a lot of times because orders are constantly under or overcut
I agree with his post.
when you have big amount of money, it will be harder reaching targeted return % rate
the money you put in a single market must be proportionally low amount compared to market cap to make it work
because if you trade with big amount, you will build a thick wall that gets under or overcut
if you do it at market price, it will look like a dump or pump, and will hurt your profit % rate
I completely agree with what you are saying, that is when this challenge will become more difficult. As of now its something fairly easy.