So? Isn't that what he, like any typical defensive general, was supposed to do: hold on for dear life? Nothing special there. It is a lot easier to defend than to attack. Depending on how entreched a defensive position is, an attacking force often needs to outnumber a defensive force to succeed. Napoleon's forces was equal in number to Wellington's forces and Napoleon still almost won.
Big deal. The French did not have the Grande Armée with them at Waterloo. The best French troops already perished in the Russian winter, to Spanish guerillas, and to Austrian troops. Furthermore, the Pennisular War veterans were never that good as British fanboys like to pretend; they did not have to face the best French troops, after all.
Furthermore, the "reluctant" Dutch troops is nothing more than a myth disgustingly perpetrated by British fanboys. It was Chassé's Dutch forces that attacked and prevented one wing of the Imperial Guard from breaking through while the British forces on the spot fled. There were also reports of British cavalry cowardly refusing orders to engage French cavalry.
This either proves that Wellington is not even a good defensive general or, more likely, it proves that Wellington was forced to give battle even if the Prussians did not promise to come to his aid. He had no choice but to give battle because he knew that the faster French forces would have caught up to his army and destroyed it if it tried to escape to Britain.