The author just posted a 10 year summary [0]:
> 1 employee (me!)
> 1850 customers
> 3.0m HTTP requests to the commercial gem server per day
> $13.5 million in total gross sales
It's a success in my book.
0: https://www.mikeperham.com/2022/01/17/happy-10th-birthday-si...
I think the big difference is that it's not really Open Source (you get the original server-side PHP+MySQL code, but only the bundled React+TypeScript UI).
The model is more similar to "old-school" software where you paid once and could use the problem, the only difference being that the backend code is easy to read/modify, and it's not a black box.
Sidenote: If you call it "open source" you'll definitely get a lot of hate from a vocal cadre who want "open source" to only apply to copyleft/BSD/MIT/etc. licenses. (I assume you're not using one of those licenses, because otherwise someone could just host your code as a free download).
If you don't want that then you need a "source available" license, not an open source license.