So you get $74 in rakeback a day, but obv that's only your profit per day if you're a breakeven player. You would add/subtract from that whatever your winnings are from the tables themselves, and what people are trying to say is that it isn't reasonable to expect that you maintain your winrate from 10NL at 50NL. At 50NL, there will still be fish, but there will be fewer of them. The regs will be better, they will not always fold to your c-bets, they instead will c/r you, they won't set-mine out of position in 3-bet pots, etc. Also, 16-tabling with only 20 buy-ins is not very good
bankroll management, it would not be unreasonable to see all of that go in one session. I'm sure the smallest amount that pros play with is 100 buy-ins. Most have at least 200 probably. And if you do plan on playing 16 tables for 8 hours a day, I would imagine that is very physically and mentally taxing. There's no way you could play an optimal game for that long, further lowering your winrate.
So yeah, you could play 50NL, but considering your experience, with fewer fish and better regs, a risky bankroll and an insane amount of volume, it'd be very hard to succeed, which is all anyone is trying to say.
btw this is fun to watch, i'm sitting in front of my computer like this: