The end of slavery and making former slaves US citizens were added to the constitution by the 13th and 14th amendments respectively, not by a change in culture or circumstance. This would be an example of the federal government abiding by the constitution and doing things the right way.
Embracing federalism doesn't mean sacrificing the Union or going back to anything except more liberty and more responsive government. If where we are is where we want to be, why doesn't the federal government do things by the book? The FDA, EPA, Dep of Education, Energy, Agriculture, Social Security, Obamacare... all unconstitutional. If we want these things so bad, why weren't amendments added to make them constitutional? Should be pretty easy since they're so popular and everybody thinks they're such a good idea.