Okay, so I went and looked up Eldred vs. Ashcroft case which challenged the constitutionality of the Sonny Bono Copyright Term Extension Act, which set the copyright extension to death of author + 70 years (a ridiculous amount of time). Basically, the SCOTUS decided that "limited Times" means "not infinite" thus any number less than infinity will do. 200 years? 1000 years? No problem!
The founding fathers were suspicious of monopoly. They by "limited Times" I feel pretty certain they didn't mean "not infinite" but rather, "a short period of time." The first part of the copyright clause is "To promote the Progress of Science and useful Arts." Yes, part of this is to allow exclusivity to financially motivate people. But the other part of that is to allow arts and inventions into the public domain so they can be used freely for improvement and creation. In no way is life + 70 years in support of promoting progress.