If you don't potty train a child then nothing bad will happen.
Eventually when the child gets to near 5 years old at most they will or should eventually decided to stop wearing diapers and start using the toilet.
I had one kid of mine that is a boy who took until he was 6 years old until he was finally done with diapers and started using the toilet.
I never really pushed it and he is home schooled but he just was not wanting to use the toilet and I did not want to make it a battle.
His brother that is younger than him potty trained on his own when he was 3 and 1/2 years old.
I introduced the potty to both of them but never forced the issue.
I figured that it was not worth the struggle and diapers are made for all ages.
I had no issue with changing the 6 year olds diapers and would much rather change diapers on a 6 year old child than to deal with tantrums and arguments.
Children will eventually go to the toilet on their own even if you do not potty train them.
But you should at least introduce them to the toilet and all kids develop at different ages.
Most kids are potty trained by 18 months to 2 years but some kids take longer.