Five years ago, Mario Draghi made remarks in London that have changed the world.
When he spoke in London on July the 26th 2012, the Euro area was in its deepest recession since WWII* and two important countries, Italy and Spain, were following a non sustainable trajectory that can be described as a deep recession with high real interest rates.

These problems came from the European decision to follow austerity policies. These latter dramatically reduced domestic demand but without having a strong impact on public finance. So the main question for Italy and Spain was the date of their exit from the Euro area; not if but when.
An exit from these two countries would have led to a collapse of the Eurozone. This would have destabilised the world economy.

Five years ago, in London, Mario Draghi did three things

First he said that the Euro Area was a political construction. It was the result of countries' will to live together. It has worked. The area has been in peace since WWII. This is the most important point for Europe. The euro currency is just a technical commitment. An important one but not more than that.

The second point is the following: if the currency is just a technical mean to improve the way the Euro area political framework works then it has to continue. The collapse of the Eurozone (after an exit from Italy or Spain or another country) would have been a source of political instability in Europe and probably the end of most European institutions.
The role of the ECB was to save the Eurozone by avoiding a collapse of its currency.
The famous sentence "whatever it takes" (the whole sentence is "Within our mandate, the ECB is ready to do whatever it takes to preserve the euro. And believe me, it will be enough.") explains this new mission.
After this sentence the ECB became the lender of last resort that a monetary construction needs. Until this moment the central bank had never played this role. It's a fracture in the European institutional framework.
In a situation where governments didn't know what to do, the ECB became the leader under the auspice of Mario Draghi.

The third point came few days later at the ECB monetary policy meeting with the creation of the OMT which allows the ECB to buy assets (public debt) mainly in Italy and Spain. This was enough to reduce tensions on these two countries. Later in 2012 and after in 2013 the interest rate spreads with Germany decreased dramatically. This was the end of the main divergence within the Euro area.

The ECB has been a game changer by avoiding the collapse of the European construction. Since then, economic policy leader is the central bank under the impulse of Mario Draghi. Progressively the fiscal policy has converged to a neutral stance. The Eurozone economic policy is now done in Frankfurt more than in Brussels.
The QE strategy was consistent with this new ECB framework. It had to create the necessary impulse that would provoke the recovery. It has worked with all the instruments used in this non orthodox monetary
The European Central Bank has been the channel for a smooth adjustment in the most important financial crisis since WWII.
Its job is now almost over as the growth recovery is now strong. It just have to keep its accommodative monetary policy in order to ease the needed political adjustment. The next step of the Eurozone institutional construction is political. It's time for government to take the relay.
We will forget a lot of ECB presidents, past and future, but Mario Draghi will remain as the one who save the European construction.

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* The Euro Area has started in January 1999, but if we look at the economic situation of constituant countries since WWII none of them has had such a recession.