Well, for me the point (essence) of promotions is for the casino to promote themselves to get more "new" gamblers to join the casino. Some of the promotions are there to get existing gamblers to gamble more. (Races/Championships/Tournaments/Challenges)
So, if the RTP is too high to pay for all of this, you will get new people to gamble and your existing gamblers will eventually get frustrated and they will leave. It turns into a cycle that will eventually "kill" the casino... because you will not keep the doors open, if "new blood" does not join the casino and the old gamblers leaving.

Right. There needs to be an equal stress on the importance of promotion and RTP. But ultimately, RTP is what carries a platform's profits in the long run. If not for RTP, casino player base might see a steep drop as time progresses. Newbies are only there for the bonuses. Once that runs out, they also hop in to another casino for the bonuses. If you can give them an incentive to stay and play for long, they'll do that on themselves and they would need not the insistence of the platform.
Well, the newbies just boost their game play time with the bonuses, but they are not the gamblers that are bringing in the real money. The profit for the casinos come with the highrollers and that is why most promotions and challenges are geared towards giving back to highrollers to keep them gambling.
The thing is... even the highrollers wants value for their money ....so a favorable RTP will also benefit them. A lot of the casinos are just greedy and they forget the basics, because they are too focused on aggressive marketing.

I also do casino hopping at times when I have the luxury of time, because who would want to ignore 250% first deposit bonus and other cool stuff that they brought upon new users? I gamble mainly for entertainment, much less about winning (because I already accepted that I'll never win big from dice, ever), that's why I always look for platforms that give my money some value and give their players some incentives if they play for long. It's what people like us look for, not fancy bonuses that will run out once you lost all your first deposit.