The first term was 14 years, not 18, and today’s life-plus-70 extensions run completely against what the framers intended.
Many of the amendments do too.
12th, 13th, 14th, 15th, 16th, 17th, 19th, 22nd, 23rd, etc.
They all change how things worked. Things the framers set up. Things that the framers intended.
However, at the same time, the framers did provide a way to change COTUS via amendments. So, arguably, the framers intended for people to make the country work in a fashion other than what the framers intended. The wisdom of making such changes is open to debate.
Since a "change mechanism" was clearly intended, and the framers clearly understood that it could be used to make COTUS work in a way other than they intended, I don't think "run completely against what the framers intended" is a very strong or persuasive argument. Those men were wise enough to understand that they were not gods and that time would demonstrate they may have gotten a few things wrong. Maybe copyright is one of those things. Maybe they would agree that it should be 70 years plus the life of the copyright holder, or whatever.
I'm not arguing that the copyright situation isn't a problem (or that it IS a problem), only that the argument about running against intentions is not a good basis for addressing the issue.
Your point is valid; however, you've conflated two related but distinct concepts: Constitutional changes, e.g., amendments, and Statutory changes, e.g., laws and regulations. Constitutional changes have a rigidly defined process for approval and are expected to be difficult, time-consuming, and carefully considered before implementation. Statutory changes are much more flexible, although they are constrained by Constitutional limits. Copyright straddles these two concepts: it's an explicitly authorized Constitutional function, enacted by statute and regulation. The "limited times" is the Constitutional constraint. Legislation -- laws and regulation -- cannot legitimately override a Constitutional constraint. Therefore, Congress' continued copyright extensions, in some cases retroactively, such that they never expire are violating not only the unwritten social contract but also a plain text reading of the Constitution. SCOTUS has thus far punted on the question of what constitutes "limited" and where Congress should draw the line; but in my not so humble opinion, we crossed that line the first time Congress retroactively extended copyright on existing works.
The Framers' intent here was clear, and until we amend the Constitution, it must be respected in Statute.