I remember knowingly buying a locked iPad. Even figuring out the carrier it was locked to was difficult (why isn't that displayed on the system information screen or on the error when you use a different SIM?) and Apple were of no help either. I bought it because I knew this bullshit and decided to go through it anyway but it isn't a pleasant experience and shouldn't be considered normal.
There's also the issue of recycling and e-waste. You're telling me to buy new, which is fair but what about the countless locked devices that are perfectly functional and yet stuck in limbo because nobody can figure out how to unlock them (even if they are otherwise not stolen and the previous account was in good standing)? Should we just accept that these devices are essentially bricked and can go for scrap because it's not worth the trouble to unlock them?
It sounds like you're talking about buying a used phone. Sure - there's danger there. Danger which can be completely mitigated by buying at the owner's provider's store, and having an employee look up that info before giving over money.
If you want to avoid all that business, it's really, really easy to just buy from the manufacturer. If you do that, they always come unlocked, not stolen and not banned.
Broadly: locked phones suck. I would never buy one (unless it was used and on a network I'm already with). But I also know several people who cannot save money. At all. Subsidized phone plans are specifically tailored for those people. The subsidized experience will always be worse than strict ownership simply because incentives between the provider and the user are not aligned. But for those people who can't save money, I think this is the best option they have.