If the Electoral College is abolished, or changed like this we might as well abolish states. They will be essentially meaningless, and we can one big collective like the USSR.
Does that mean the only purpose of having states is to elect the president?
What you should ask is: What is the purpose of the Federal Government? Not, "What is the purpose of the States." The purpose of the Federal Government is found in Article I+ II, Article III and Article IV:
Article IV, Section 4
The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.
Article III (Judiciary) clearly states that the federal Courts are to settle disputes between states.
Article I+II allow the federal government to use military force to suppress insurrections, rebellions and to enforce express provisions of the US Constitution (for instance the feds could use military force against Long Island if it tried to secede from New York without Albany's consent due to the express constitutional provision that no segment of a State may secede and erect its own boundaries within a State defined and protected by the Federal Constitution).
The Congress is the body that decides whether or not to use military force against enemies (foreign or domestic) and the President is the one who decides to use such forces (Command in Chief), thus the States that assented to the Constitution did some to gain these protections from the Federal Government; however, assuming only that they have an equal voice in how the Federal Government proceeds:
The small States asked for a Senate.
The large States asked for House of Reps.
The Founders compromised and gave us both, checking popular and state powers.
Future states, small and large, applied to join the Union, since each had an equal voice in legislative power and choice of the executive (President). States with tiny populations get 3 Electoral votes...giving them more of a voice then they would have under a popular system, but no where near enough of a voice to upset the balance of the more populous States.
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Democrats are willing to turn a blind eye to how the Constitution came to be with its series of compromises between large and small states, and separately, Federalists and Anti-Federalists.
The same arguments and reasons for these compromises between the small and large states remain valid today: Smaller states (by population, but equal or larger landmass) will not tolerate their voice being nullified by larger states (by population with same or lesser landmass).
It's these compromises that created the Constitution as we know it and made both small and large states (by population) comfortable with joining the Union. Undoing these compromises without the consent of the smaller states (with amending the Constitution with the consent of 3/4 of the States) in order to benefit the more popular states (cities to be honest) will only lead to mass secession and civil war.