If you don't want your software to change, don't update it.
The way Thunderbird looks and the messages list is a large part of it is horrendous and is a deal breaker for me, for example.
It is an application that receives content from the Internet, your advice is dreadful. Email is a primary vector for exploitation.
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.