power of ICANN and therefore power over all the internet
ICANN has very limited power over the internet.
Telecoms companies and ISPs have ultimate power over the internet.
Nobody can force those entities to use ICANN's addressing system. It's a purely voluntary agreement.
If ICANN starts screwing up the internet too much, those entities will stop using ICANN and find an alternative address assignment organisation.
In the worst case it might lead to a fragmentation of the internet with several independent IP address spaces and domain name spaces.
ICANN knows this too and will be careful to prevent this from happening.