The best bonus is not to take the bonus if it has a big wager, because it will be -EV (-Expected value). Any casino game is -EV, if you play by the rules of the casino. I will give an example of why you should not take deposit bonuses with a high wager. The casino gives a 100% bonus on the first deposit and wager x40 (D+B) = 200 x40 = 8k, we make a deposit of $100 and they give us another $100 = $200 (wager 200*40=8k). The casino has an average RTP of 96% (percent is different everywhere, depends on slots or house games and so on). For every $100 wagered we get back $96 according to the mathematical expectation (100-96=4), $960 for 1k wager (1000-960=40), $7680 for 8k wager (8000-7680=320). And after the full wagering, we will lose $320 (D+B and also we need more $120), which gives a very large -EV. Who thinks about this and the casinos began to raise the wager.
It's true that all casino games are negative EV for the player and it will be very unlikely for us to win constantly at the games. The longer we play the more likely is it for the casino to win. Which is not guaranteed though, the negative EV means that on average the number of losers at the casino will be higher than the number of winners. That's how the casino makes it money, but it's not a guarantee that there will be only losers. Some gamblers will be lucky and make money. As for your calculation the deposit bonus is only, and with a x40 wager requirement it would mean we need to wager 4k USD. The 96% RTP is not specific, with roulette or a good blackjack strategy we can get higher winning chances. So even with the lower 96% we would have 3840 USD after watering 4k, which would leaves us with 40 from our 100. And let's say we play more optimal with 98% RTP, we would make 20 USD profit. Also most deposit bonuses are 200%. So far k had good experiences with deposit bonuses, just don't focus on instantly chasing out again, eventually you will hit the barrier to withdraw.
You've a hint of truth regarding casino bonuses, yet a deeper grasp of casino math is vital. All games being -EV is, broadly, correct but remember, it's an extended period average. An individual's reality could differ greatly. Your RTP point, around 96%, overlooks a key fact: RTP reflects millions of spins, not one game. So, one could still score a big win short-term, RTP notwithstanding. Moreover, discussing expected losses, wagering requirement and bonus size are key. A smaller requirement could, in theory, make a bonus worthwhile, even with a -EV game.