Actually, if we talk about casino games (ignoring for a moment sportbetting), bankroll management is the only factor in our session on online casinos we can control as gamblers, because the seed and entropy are completely random and outside of our reach, that is something not all gamblers understand, specially those we are new in this ecosystem.
Anyways, we should also be aware bankroll management is not about managing our money to get profits from the casino or something similar, it is rather a way to extend our session and make our money to last longer so we can have more fun and thrill with the same amount.

In the long term, the casino will take it all, as it has the house edge on his side.

This is actually one of the hardest parts while playing in an online casino. Slots and casino games. To prolong the playing time. Why? Because I feel like the house edge is getting higher once they are licensed, especially in local online casinos.
I have played slots for a long time, and I have never seen myself playing hours with just my minimum deposit. It always just takes minutes, and it's all gone even though I am betting for the minimum amount.

Plus, they also do not reveal the RTP anymore, so it's more likely that our funds will be gone so fast. I guess part of bankroll management is also finding the right game that gives back using a good percentage of RTP.

You said it all, bankroll management is the step to be qualified to be a responsible gambler, but this is hard to achieve because you would always be tempted to chase your losses at so point and if you are not well disciplined you will fall for this. You can set aside an amount of money that you would use to gamble and plan not to go beyond it but when losses starts coming in without discipline your plans might change