Guillaume of course you can do this and Apple surely is the first of the big tech companies I'd trust my data with if I'd really had to, based on their track record alone. Fortunately I don't have to and find myself comfortable in a position where having all my sensitive data offline or on GOS is possible and convenient for me.
I also agree that open source doesn't mean more secure. It only allows for better control and prevents shady stuff such as backdoors to be implemented. For that to happen, you need many professional eyes on the code though, so I'd only trust popular open source code vetted by experts.
Now with the marketing: Why would that be a reason to trust Apple specifically? Any successful company does ads based on what they expect to sell best, not on what is actually true or useful. A high price is also no good indicator for more privacy as it's both arbitrary and profit-driven. A simple look at GOS vs iOS proves that.
Apple hires marketing experts and psychologists to make you feel safe and private with using their devices and services at a premium. Giving you true privacy will cause them lots of opportunity costs by not collecting and selling your data. Do you believe any investor driven company would make that decision?
Here are just a few examples of how Apple proves to me that they are just a tiny bit better at privacy and a lot better at selling their market consolidation as privacy feature to you. I recommend to always judge companies by their actions rather than their statements statements.
- You don't get real control over your data. Apple is currently facing a lawsuit because they keep collecting the data via their own apps that you specifically chose to not send via iOS privacy settings.
- Apple takes about 18 billion $ from Google per year by participating in the privacy invasion that is Google ad revenue.
- Apple doesn't allow you to install apps outside of the appstore which they control and collect your data with.
- Apple forces you to create an Apple ID to use your iPhone in any meaningful way.
- iCloud is collecting a lot of data and metadata by default, not all of which is manually removable.
- Apple couldn't fix a security bug in their WebKit browser engine for months, exposing all iPhone users to exploits since all web browsers have to use it on iOS and are basically Safari with a different skin.
- They have a pretty privacy website explaining how they protect or don't collect your data, while in their hidden ToU (which are the actual, legally binding privacy terms) they make contradicting statements. This is shady and misleading behaviour I can't trust.
I could go on, but instead I'll give you this video which made me lose all hope in Apple being actually private.
Again, I'm not hating Apple or saying it's not a valid choice for the right threat model. They sure are the least evil of the big ones when it comes to privacy and there are very good reasons to use them based on security, convenience, design, interoperability etc.
But saying Apple truly cares about your privacy because they charge more money and run better ads... That's not a good argument in my opinion when looking at their actions.