Then one might ask why 'necessary and proper' doesn't include legislation for Social Security, Medicaid, farm subsidies, etc. because the Constitution also is there to 'promote the general welfare?' The long and short of it either there are implied powers or there are not. If there are not, the the Air Force and God knows how many other government functions and agencies are unconstitutional, and the Constitution itself would need to be amended 15 times a year. If there are, then implied powers doesn't end where you say they end.
That's a common misunderstanding. Actually, there is no "general welfare" power given to Congress. Article I, Section 8, Clause 1 states:
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United State; but all Duties, Imposts and Excises shall be uniform throughout the United States;That whole clause is about taxes. The expression "to pay the Debts and provide for the common Defence and general Welfare of the United States" appears between two other provisions specifically about taxes. It was drafted as a limitation on the taxing power, and it describes the intended purpose of the authorized taxation. Madison discussed this in The Federalist Papers : No. 41.