Brushing your teeth for too long can have bad effects on your dental health[1][2]. The heads loose their strength over time and if you replace that with pushing harder, you might also overbrush.

[1]: https://www.colgate.com/en-us/oral-health/brushing-and-floss... [2]: https://riverrundentalspa.com/brush-teeth-too-long/

You probably don't brush correctly: you need to apply sufficient pressure on your teeth so the bristles can scrub the enamel well enough. Or else there is no way your head has no visible deterioration after 5 months at 5 min per brushing.

It's not only a matter of aesthetics but bacteria etc. Your brush head collects a sh*tload of food waste (which you don't see with your eyes) and remains a bit wet after usage. Perfect substrate for some living organisms. If you have a microscope, feel free to have a closer look.