I have hi Sierra older Mac. Will it eventiaucrash since Apple does not support after 2020?

Not to worry. High Sierra is still a fine operating system which is well proven over time. All it means when it is no longer supported is that you will no longer receive security updates. And none may be necessary. The problem you will eventually face is that as other vendors upgrade their software, the newer versions will eventually become incompatible with High Sierra.

You iMac running High Sierra will continue to run with the Apple unsupported High Sierra until your storage device is full, or fails. My 2011 Mac mini cannot upgrade past High Sierra, and it just continues to work.

Set a little money aside each month for your next Mac. Don't take food off the table or compromise other important expenses, but be methodical in this approach.