Welcome bonuses don't keep customers; they're meant to attract customers and make them taste the casino. It's their experience in the casino that will determine if they will continue playing there or if they will use another one, since welcome bonuses are mostly given just once after the first deposit. I don't consider it as something that can keep players to continue gambling there.
But seriously, Welcome bonus is dedicated only to make players entice to deposit and play since casino is confident that they will start converting users to become a loyal player once it taste an experience on the casino.
I kind of agree with your last statement; the reason for the bonus in the first place is to drag and convince people to come try out their own casino over that of competitors, and while using the bonus to lure them in, just like I stated above, the experience of that player will be what will determine if they will continue playing there or not.
If the new casino offers some good games and features which they are okay with, then the casino gets itself a new player, but if it's not, they will still lose the person immediately as the player either is tired of trying to meet the wager requirement or eventually hits it and withdraws everything.