Shopify offers a full ecom platform and let's you launch in days if you really have to. Upfront you aren't paying anything. Creating Shopify's functionality will take you years and hundreds of thousands of dollars to develop.
You did not recreate Shopify in a week. You think you did but you only implemented a custom solution that does 5% of what Shopify offers, including the expensive stuff like compliance, support, etc.
No sane business is going to use a random dev with a total custom e-commerce solution when they could throw a couple of dollars at shopify each month and get 100x the value.
This sounds interesting. I sincerely thought a shop was just a website displaying products. I got this idea to work. People can buy things. Was that not the point? I can hear you wonder but yes, I'm really this naive. You are saying I'm missing out on 100x the profit? Could you give one or more examples of this?
If we drop all of the things not useful to developers (or actively get in the way) I'm left with:
Logically, at least some of the 1500 ready made adons must do something that is hard to make yourself. The downside is that you have to pay and that writing things (not available in their app store) yourself might be hard to impossible.
They offer SEO tools which (I hear) are quite good. End of the day yours is just another shoppify shop which makes it hard to compete with others doing the same thing on the same platform. WooCommerce is argued to provide better results. I have no idea how a self made website compares.
Integration with accounting software, shipping providers and payment processors out of the box is kinda nice but one should be able to figure this out.
There are also hundreds (joking) of downsides that I'm not able to judge.
Overall it seems a good solution if your store isn't to small and isn't to big. It might be fun using it to buy time while you transition from small custom to a large custom solution.
Most ecomm businesses are more complex than connecting a front end to a payments gateway. Generally requires multiple complex systems (Product Management System, Order Management System, CRM, etc) to work together seamlessly. This is where platforms like Shopify or Magento are really valuable. They give you all the pieces you need to build your business w/o needing to reinvent the wheel.
Don't forget inventory management, warehouse management, shipping etc.
Often platforms do a little of these themselves but lots of businesses have needs that require them to use specialised solutions that handle those aspects of their business better than Shopify does as each is a complex domain in its own right.
Stripe is only good for payments. Square does storefronts, but I have no experience with them. Shopify is “good enough” if you need to get a storefront up quickly, and integrate with payments and shipping.