There is no reason to believe that a divested, independent app store(s) would have lower quality control. In fact, they would have maximum incentive to triple-check the apps against malware and spyware etc.
No, you fail to grasp what else the App Store does. Apple also aides the developers by developing the APIs that interface the hardware, operating system with the apps. It is impossible to do that without being part of the development of the hardware and hardware from the beginning. If Apple were divested from the App Store, then any other competitor app stores would demand equal access to Apple hardware, deep OS, deep API necessary for their success and any of their development to assure what you want. Apple would have to reveal security secrets that would compromise exactly what should NOT be compromised about the OS security models and methods.
There is a reason why Android Apps from independent app stores ARE lower quality than iOS apps. It’s because your assumption and assertion is wrong. That model has been tried and found wanting. The number of NEW Android malware identified per month in 2020 was approximately 530,000 or almost 7 million per year. On the other hand, Malwarebytes reported that in 2020, iOS had very few malware issues:
"On the iOS side, malware exists, but there’s no way to scan for it. Most iOS malware is nation-state malware, spread via targeted attacks through iOS vulnerabilities, such as NSO’s Pegasus spyware. It was learned this year that China had gotten in on the action as well, using iOS zero-days to infect phones in a targeted attack against the Uyghur people."
Malwarebytes went on to report that a deep seated boot ROM hardware vulnerability in iOS devices from iPhone 4 through iPhoneX, which cannot be patched, was discovered in late 2020 that could compromise a physically possessed iPhone, but it would require sophisticated equipment to accomplish. In addition, by the time of the vulnerability’s discovery, only about 5% of iPhones in the wild with that boot ROM were still in use as most users had upgraded to newer models using a later bootROM that lacked that vulnerability.
It was pointed out that the vertical integration of Apple’s App Store business model maintains this low level of malware intrusion.
So your thesis is contrary to reality. Empirical evidence in the real world does not support your wishful thinking.