Hardly adverts to be honest, in fact you could argue the last one is actually a good practice rather than just not telling you and letting you be charged.
As for the first two you can turn notifications off.
The last one isn't warning you that your trial is running out. It's warning you that if you don't start your 90 day trial soon, you will "miss out" and only be able to sign up for a shorter free trial period.
And yes, I'm aware you can disable them. My point though was that Apple definitely does do ads. Oh and by the way if a third party developer were to use notifications like those three? They'd risk having their developer account banned. It's a blatant violation of Apple's developer agreement.
Why is the fact that there are no ads in Apple apps relevant to this post? None of this will stop you from using Apple apps.
Allowing people to use non-Apple apps won't force people to not use Apple apps. People will only use non-Apple apps if they think it's doing something better than Apple apps. If, as you say, Apple apps are the best, then people will use them even if Apple allows competitors into the iOS market. I do not see a single way this decision hurts any consumers.