The problem isn't the %. It's the perpetual IP license, which grants Facebook complete non exclusive rights to the Creator's IP, including licensing it to third parties without further pay to the Creator.
> You keep full ownership of all content that you post on Patreon, but to operate we need licenses from you.
> By posting content to Patreon you grant us a royalty-free, perpetual, irrevocable, non-exclusive, sublicensable, worldwide license to use, reproduce, distribute, perform, publicly display or prepare derivative works of your content. The purpose of this license is to allow us to operate Patreon, promote Patreon and promote your content on Patreon. We are not trying to steal your content or use it in an exploitative way.
Facebook, on the other hand, has numerous ways to benefit that don't proportionally benefit the creator, including from indirect revenue sources like advertising and data enrichment that don't flow back to the creator in any way. I suppose this is alright when you are just showing cat photos to your family, but it's quite different when you are basing your business on someone else's market-dominating platform with large negotiating leverage. And there are known cases where Facebook has been known to very liberally appropriate ideas from companies that are competitive with them (e.g., Snapchat).
So if your content would make a good ad for Patreon, you've given them permission to use it without compensating you.
Google has a guarantee on when your deleted data disappears from tapes in vaults, and it was something that every engineer at the company had to think about (not to mention a ton of work in actually implementing it). You may wish for Some New Startup with 2 engineers to do it for you, but it's hard, takes time, and nobody actually cares. Look at how many people use Patreon despite that policy. Why spend years of engineering work when your lawyer can get the problem fixed in 5 minutes? It's just economics.
The real problem is when Facebook uses that content to further their business interests. Likewise, the publisher shouldn't be sending that rejected manuscript around for others to mine for ideas.
(that said, to avoid the temptation that gives rise to this precise problem, it's common practice for publishers to return rejected manuscripts)
That argument makes no sense if you're talking about publishing, AKA making something available to the public.
But the real world of production computer systems is not always the happy path. Consider the case where you delete your account, then the service suffers a database problem, and your content doesn't actually get deleted (failover to a replica, restore from backup, etc.) Now they are violating that contract with you. That is why they make the terms of the contract "in perpetuity" so the engineers can fix the production system incrementally, rather than saying "welp, there's a risk of a lawsuit if we failover to the backup, so we're shutting off the website and paying back the shareholders, everyone go home, we're done here".
Like I said, some companies take a middle ground where they really do guarantee deletion even if unhappy events take place at exactly the wrong time. It is possible. It's just not very economical.
If this was just about liability, the terms could be much narrower. They're intentionally broad to guarantee the maximum benefit to the platform and allow them to come up with other uses for that data later on.
This is exploitative and only works because of the power imbalance between platforms and individual creators. Even if you follow the idea of the "free marketplace" you'll have to acknowledge that creators have no individual leverage when it comes to platforms with "take it or leave it" ToS.
The objectionable part is not the exclusive license. The objectionable part is the sublicense right, which lets Facebook sell creators works to third parties without paying the creators, and even after the underlying business relationship with the Creator has ended.