Having to pay is by itself a filter. So, some curation is done.
On the other hand the 30% fee would stop me putting my software in an app store. I know the usual argument is they will deliver extra customers however that argument seems dubious when there are 2 million other apps fighting for attention.
People are willing to pay for those 30%, as long as iOS has an healthy supply of apps, apple will keep it.
$99 per year seems quite a lot, if you are not planning on making money with the apps.