If there's no Nerzhul and hence no LK then no one goes to Northerend when Yogg-Saron is released, so Algalon comes and wipes the planet clean without any resistance.
Actually without anybody going to Northrend to kill Lokhan "Prime Designate" who was working for Yogg with his Irons, the "Alagalon failsafe" would not have been activated and he would never have arrived.
Ok so if the Horde never arrived.
Humans would continue to war endlessly with each other, as well as fighting trolls and others. Many races would be isolationist and focus on their own affairs, gomes would still irradiate their home, tauren would be wiped out.
With the lack of the Orcs, the legion would still need to create another proxy invasion force to weaken Azeroth before their arrival, and would pick something else.
Medivh would still be possessed by Sargeras and would simply hatch a new plan.
Broxxigar, Ronin and Krasus being sent back in time created an alternate timeline (no idea if it overwrote the original one, or created a split-off), but in the original one, there was no Broxxigar, and the Legion was still defeated.
You don't actually have anything to back that up besides your own supposed.
Since its been made cannon law, the timeline only worked because they travelled back. Time travels funny that way, events only occur because of time travel itself in such a fantasy environment as this.