because some dont get security and feature/bug updates. for that i recommendeed simply to install something like e/os as an operating system (OS). It sometimes still support 10+ year old phones and likely continue to support them for a while.
The tech started to get so good that old phones are good enough so refurbishing them with another OS and not to trash them like profit focused companies want.
It depends. Ironically, cheaper phones are more likely to still have headphone jacks. Pretty much most entry level phones have headphone jacks, but they suck in literally every other way...
I bought my current phone for about 100 euros over a year ago, and it works just as well as my previous one which was much more expensive. Also has headphone jack.