If the Constitution is just an enabling document, rather than a limiting one, what do we use all the men with guns for, and how did governments without foundation law function before it was written?

In Anglo-American legal tradition, an explicit enumeration of included elements is an implicit exclusion of anything else. This is contrary to common sense, where if you say my grocery purchase includes apples and oranges, you might also be buying bread. Not so in law. If you write a list that includes apples and oranges, there is nothing else; that is the entirety of the list.

When the Constitution explicitly enumerates the powers of the government, it is saying that these things are the only things this government is allowed to do. In practice, the government found the vaguest and broadest enumerated power, and cites it as its justification for everything else that it does.

With the Bill of Rights, the 9th and 10th were inserted to allay those fears. Read the 9th again: The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people. That is an explicit reversal of the general legal principle that an enumeration is a complete list. Otherwise, it would indeed mean that the people have no other rights beyond what were enumerated.

So it is both enabling and limiting. The two are not mutually exclusive. But as the 9th amendment shows, there was a default assumption in place that requires the 9th to be inserted to explicitly contradict. Those people that argue that it is an exhaustive enumeration are most likely correct. But de jure and de facto exercises of power are sometimes different.

If you enumerate a power that says "...and anything else the Congress decides to do", then there is no limitation. The government doesn't even need to bother with the enumerated powers. Just throw everything into the catchall clause, even if it could be justified elsewhere, just to save time. And that's what the interstate commerce clause has become. An overly broad interpretation of that one power renders the remainder of the document largely irrelevant.

That's why the Bill of Rights was necessary. The Antifederalists thought (correctly) that the federal government would grow well beyond its initial limitations. Madison was aware of their arguments, so in addition to the whitelist of enumerated powers, there had to also be a blacklist of enumerated prohibitions. And it had an explicit declaration of "plus other stuff we didn't explicitly list here".

It should be a stark contrast. The main body of the document was written by and for optimistic Federalists. The Bill of Rights was written as a failsafe mechanism to address the concerns of the Anti-Federalist opposition, for when the original document inevitably failed to contain the unrestrained exercise of government power. Some people predicted before the thing was even signed that it would fail at its weakest point, which just happened to be the interstate commerce clause (but if it wasn't that, it would have just been something else).

The fact that we are now operating largely on the failsafe backup mechanism is a concern. We have no backup for the backup. The smart thing to do, rather than argue about the right to bear arms for illegal immigrants, is to patch the original point of failure. Fix the interstate commerce clause to make it more painful to use as a catchall, or repeal it entirely.