1. If in buying and writing option, I always hold to expiry, I shouldn't care what future volatility expectation is but future underlying direction and value? Since I already got paid (premium received), whether I make money or not depends on where the underlying lands at expiration?
Sure, but usually when vola rises your underlying "lands" out of the money. Plus you'll probably also scare your pants off and make new decisions based on the risen vola.
2. An ATM straddle is essentially delta neutral, or close to it. Then by constantly adjusting to keep it delta neutral, should OP receive risk free rate net of commissions and slippages?
You are referring to classical delta hedging, something the market is already keen on doing. OP will profit from it only if he's better at delta hedging than the market. Is he ? Based on his first post, No.
3. In today's zero interest rate environment, such a strategy will not be very profitable?
What does interest rate have to do with the profitability of this strategy ?
Why do you guys still think that there somehow exists a golden goose strategy that requires zero effort and nets a sea full of coins ?
You do realize that someone literally has to give you the money if u want to profit. Now, why would he do that ?