How these companies don't hire kids like Daniel for pennies on the dollar and have him attack their stacks on a loop baffles me. Pay the kid $50k/yr (part time, he still needs to go to school) to constantly probe your crappy stacks. Within a year or two you'll have the most goddamn secure company on the internet - and no public vulns to embarrass you.
It's not quite that simple. I don't think most bug bounty participants want a full-time job. But even more-so in my experience they are not security generalists. You can hire one person who is good at finding obscure XSS vulns, another that's good at exploiting cloud privilege escalation in IAM role definitions, another that's good at shell or archive exploits. If you look at profiles on H1 you'll see most good hackers specialize in specific types of findings.
I've never heard an XSS vulnerability described as a supply-chain attack before though, usually that one is reserved for package managers malicious scripts or companies putting backdoors in hardware.