It’s not bad design, it’s definitely intentional, however I agree that it’s probably not for having backdoors, but for convenience. Average people forget their passwords all the time, and with encryption that level of carelessness is fatal to your data if they have not saved it somewhere, which they probably didn’t do.

Very few devices are rooted and usually you cannot get root without fully wiping your device in process.

I’m pretty sure the system is not flawless. Probably it’s harder to find an exploit in the OS than it was years ago, but I would be surprised if it would be really rare. Also, I think a considerable amount of people use the cheapest phones of no name brands (even if not in your country), or even just tablets that haven’t received updates for years and are slow but “good for use at home”. I have one at home that I rarely use. Bootloader cannot be unlocked, but there’s a couple of exploits available for one off commands and such.