I offer both in https://kidzfun.art. If you're non-technical, you can buy packs of 100 images and it uses my key to access Dall-E, or you can provide your own key and pay nothing to me. The vast majority of users go the simpler way, but it's a nice bonus for technical users to just reuse their own key. The difference with your approach is that I store an encrypted copy server side as I do all the AI generation on the server.
Not saying it isn't reasonable, but I'm guessing people might downvote because of storing secrets server-side rather than passing them on from the frontend and saving them there instead. People get worried as soon as secrets are stored anywhere :)
I think the biggest issue is that the vast majority of all Internet Users, including 'techies' really dont understand Secretes, Security, risks, non-risks etc...
I think that What HN (the site) is actually lacking is any kind of formal education [section] on the state of tech. Esp. given how much of SV tech zeitgeist flows through the frontpage of HN and the folks in its orbit - HN is missing out on a service that could look like a "tech News podcast" where Khan Acadamy meets OpenCourseware CS level snippets...
As an example - there have been a flurry of tools and launches and shows to HN recently that if there was a 15 minute video explaining the TechLego - and you could watch all these announcements and little educational doo-dads for the various tech componentry and tooling being shown here - a scrappy motivated modern version of 20-year-old [Every Grey HNer] could build wonders with...
We need to give people a solid grasp of all these concepts and issues, best practice, and the WHY we think the way we think about things such as secrets, auth, security. (the boring layer in OSI for most)