With Dooku and Grievous dead and the Droid Army driven into the Outer Rim, Palpatine was ready to execute the final stages of the Grand Plan. Regardless of whether Anakin joined him or not, Sidious would have executed Order 66 and wiped out the Jedi. The justification for Order 66 was just something that Palpatine used too placate the Senate. The official description of Order 66 reads: Order 66: In the event of Jedi officers acting against the interests of the Republic, and after receiving specific orders verified as coming directly from the Supreme Commander (Chancellor), GAR commanders will remove those officers by lethal force, and command of the GAR will revert to the Supreme Commander (Chancellor) until a new command structure is established. Notice that it never mentions any justification or a Senate vote. If he wanted, Palpatine could wake up, get his coffee, check his stocks and send a pre-recorded message to all clone commanders, telling them to execute Order 66. And they would do it, obediently and without question. Palpatine wouldn't even have to meet a Jedi that day, he could issue the order at any point during the war.