In the debates surrounding the Constitution, the fear of a standing army was one of the greatest concerns the Founders had. The fact that there is a standing army sleeping at the Capitol Building would have concerned them more than that a few protesters marched through the Capitol.
Excerpts from various documents surrounding that debate:
The Pennsylvania Herald on Oct. 17, 1787: “Should [the general government] ever pretend to tyrannize over the people, their standing army will silence every popular effort, it will be theirs to explain the powers which have been granted to them . . . [James Wilson] told us that a standing army, that great support of tyrants, not only was not dangerous, but that it was absolutely necessary. – O! my much respected fellow citizens! And are you then reduced to such a degree of insensibility, that assertions like these will not rouse your warmest resentment and indignation? Are we then, after the experiences of past ages, and the result of the enquiries of the best and most celebrated patriots have taught us to dread a standing army above all earthly evils, are we then to go over all the thread-bare common place arguments that have been used without success by the advocates of tyranny, and which have been for a long time past so gloriously refuted!”
Alexander Hamilton, in the Federalist VIII: “[Standing Armies are] the same engines of despotism, which have been the scourge of the old world.”
David Redick (Vice President of Pennsylvania), Sept. 24, 1787: “Why will [Congress] have power to keep Standing Armies in time of peace?”
Samuel Bryan, Oct. 5, 1787: “A standing army in time of peace [is the] grand engine of oppression.”
Samuel Bryan, Oct. 24, 1787: “This grand machine of power and oppression may be made a fatal instrument to overturn the public liberties.”
Arthur Lee, November 29, 1787: “Who betrays so little knowledge of ancient and modern history, as not to know, that some of the freest republics in the world, never kept up a standing army in time of peace? Can you, O deluded men, not see that the object of all this is to fix upon you, with your own consent, a strong government that will enable a few proud, intriguing, aristocratical men to trample upon your privileges at pleasure?”
George Bryan, Oct. 12, 1787: “It will be made the power of Congress to raise and maintain a standing army for their support, and when they are supported by an army, it will depend on themselves to say whether any amendments shall be made in favor of liberty . . . It was a celebrated observation of one of our assemblies before the revolution, during their struggles with the proprietaries, that ‘those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.’”
I could go on, but this gives you an idea. The founders were not fans of a standing army. They would be turning over in their graves about what has happened at the Capitol this month, but not because of what happened on January 6. It would be what happened in the weeks since January 6 which would arouse their suspicion.
(Source for all quoted material: Bailyn, B. (Ed.). (1993). The debate on the Constitution: Federalist and Antifederalist speeches, articles, and letters during the struggle over ratification Part 1 September 1787 to February 1788. New York, NY: Library of America.)