That's not necessarily true. Your PnL will be path dependent based on the direction on the moves and your hedging. That's true even if the stock has realized higher than ambient implied (the whole premise of "free gamma before earnings"). If you are stating that you are making most of the money on vega (*) the fact that you are losing vega on your strikes can not be positive. Unless I am missing something, like you are saying that the trade is to make money on gamma and vega gains are secondary.
(*) I dispute the whole vega ramp-up story in a modern market-maker environment. Every market maker has a statistical model of earnings and other recurrent events and adds special weights to these days. Yes, "nominal" implied volatility does increase before the earnings but that's just because ambient days are falling out. E.g. if you have a 10 day option with a 10% event day and 9 2% ambient days it will have an implied roughly equal to 59 and next day, as long as event is still upcoming, the implied will be a bit over 61. That's simply because the event is dominating the implied. You see that especially strongly in biotech stocks where the term structure is sometimes pricing a single 20-40% daily move.
As a general rule, when the stock moves away from the strike, the straddle should benefit (assuming the straddle started at ATM strike). You are correct that if at the same time IV decreases, you might not see much gains. But this rarely happens before earnings.
What I'm saying is that those trades can make money from vega or gamma or both. And if the stock doesn't move, in most cases IV increase will still cover or slightly outpace the negative theta.
What you are saying is that the markets are efficient and the prices reflect the upcoming earnings. This might be true in theory, but not in reality. Just look at recent cases of AMZN, GOOG, MSFT etc. where the stock moved double of the pre-earnings straddle price. You call it single event? Those "single" events would erase months of gains if you followed tastytrade advice and sold premium before earnings only because "IV rank is high".
And if you want recent example of vega gains before earnings, look at QCOM where we entered the straddle at 2.43 and exited at 2.77 within one hour, and the price continued climbing and reached 3.00+ within the next few hours. IV increased from 65% to 80%+.
You are making a claim that front-month straddles pre-earnings are (a) systematically cheap (in 80% of cases?) 2 days prior and (b) in most cases get re-priced before the earnings move. Plus, supposedly (c) that phenomena exists in the most liquid optionable stocks and (d) is sufficient to cover round trip transaction costs.