You didn't erase them; you merely backspaced. Going forward will overwrite the previous characters, but backspace doesn't erase as it backs up. You would want
print "I print Backspace !"
... to see ...
I pri Backspa !
If you want the "full effect", you have to backspace and overwrite wtih spaces as you back up ... then you can move forward. Something like
print "Backspace" + 2*" " + "!"
You can use the multiplier as many times as you wish; it's a small motif. The above line displays
Backspa!
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