Correct. And also customer needs, UI technologies, the way people use apps and services change. So it's all right for apps to change as well.

It's alright if the UI isn't made worse.

The modern example of fuckhead UI changes. Changing settings without having to click apply and/or OK. Changing from the "cog" icon to those three idiotic bars (hamburger?).

Interfaces are often used by people who are not IT savvy. The simplest of UI changes can mess their decade(s) old patterns of usage, the elderly are the easiest example of users affected by this. It not just their individual problem, those people then reach out for help which can be at considerable personal financial expense.

Shouldn't mailservers and desktop AV be doing the heavy lifting on this front and blocking known exploits?

For your purposes I'll keep it simple and just say "about 99.9% yes". Trust the word of a seasoned veteran - keep email clients, browsers and OSes up to date.