"51% attack" is the name of the attack, but you don't actually need 51% of the hash-rate to do it, just 50% plus a small margin.
With so many actors it could be very steep. Like 51% capacity has 99% chance of success and 49% has 1% of success and 40% has practically zero chance (these are made up numbers).
It could also be quite shallow as in 1% capacity gives you 1% of success... that would actually be quite bad and invite malicious actors to test the system with regular attacks.