The reason I have a problem with this is that they add zero value other than being a central repository. At least with the App Store you are guaranteed that the app has no malware and has been reviewed for compatibility, stability and some level of quality.
It smacks of monetizing for the sake of monetizing.
App stores have tons garbage quality apps, predatory apps and apps that crash. Sometimes even malware if it's sneaky enough. The process of checking them is largely automated afaik. At least the apps I have worked on have been approved for release way too quick to have anyone vetting them.
The value of a central repository for creators is discoverability and for end users convenience, provided it's not run purely by a popularity system where only the most popular makers are seen.
As an example if I wanted to start selling IRs or captures, I could set up my own website (or something like Shopify), with my own shopping cart/payment system and try to post on various forums/social media "hey I make these" and some people would be interested. But the vast majority of users are not necessarily visiting these.
More likely they would go to the QC marketplace and search/browse things because it's The Official Way. If buying what they are interested in is easy enough, they will choose that route over having to make an account into yet another website, wait for an email delivery of the goods, having to unzip and install it on their device etc. Now compare that to click, buy, install ease of the typical app store experience.
The typical end user is not really going to care too much if it's the only place they can buy stuff as long as it's easy and convenient for them. Just look at Apple's iOS App store. Despite sideloading capabilities, most people aren't doing that on Android either.
Yes, NDSP absolutely see a marketplace as an opportunity to monetize. To be fair the storage, data transfered, payment integration and running the cloud backend does cost them too. This + profits will be supported by the people who buy paid content from creators but there will still be a lot of free content typically.
The future is "I'm bored, taking a dump in the office toilet, why not buy a pack of captures on the QC cloud to treat myself when I get home."