Section 8 of the Constitution explicitly allows Congress to protect copyright.
> To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;
DMCA has withstood a lot of 1A tests in court, even some very specific and narrow fair-use claims. The suggestion that the entire law is infringing is simply unfounded. There are many limitations that the court allows on speech under the 1A, and most of those aren't even listed as exceptions in the Constitution, as copyright is in section 8. As unpopular as it may be, the constitutionality is pretty solid.
> Repealing DMCA
Be careful what you wish for. Repealing DMCA would mean repealing Safe Harbor provisions, which would just make it just about impossible to run websites with user-submitted content. DMCA might have some problems, but do you really want website owners to be held liable for everything a user posts on their site?
> Since section 8 exists DMCA is nearly entirely redundant.
Absolutely not. There is zero overlap between the two. Section 8 doesn't establish any copyright law at all. It merely establishes Congress's right to regulate it.
> Repealing DMCA would do nothing to strip copyright owners of their rights.
That's a pretty loaded statement to make. There are obviously things that rights owners couldn't do if the DMCA was repealed. I think what you're really arguing that those things don't matter? That could be a valid opinion, but it's still irrelevant. If Congress has the power to regulate something, they also have the power to regulate it ineffectively.
This actually kinda makes sense, and the argument goes like this. It is rather obviously subjective what constitutes "promoting science and useful arts" (including the very definition of "useful" etc). So when it comes to enforcing this clause, you need some body to decide on what these translate to in practice. And this can be either the courts or the legislature. Between the two, the legislature is the one that citizens have more control over, so giving it the ability to decide is intended to make these kinds of abuses easier to rectify (just vote for the right guys!) as opposed to dealing with obstinate courts.
Let me assure you that you haven't discovered something hundreds of years of legal scholars didn't yet realize. You are simply mistaken as to how that clause is interpreted within the context of the rest of the document ... part of which outlines the division of roles and responsibilities for judging and executing that clause.
All of that aside, "science and useful arts" never referred to the little guys. This always referred to the interests of commercializing those endeavors.
> To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;
The 1st amendment didn't repeal this.