Thats a tough call. Would you redirect the old pages? If so where would you redirect them to? Redirecting to the home page is not a good idea. I would be cautious getting rid of content that has links to it without having somewhere to redirect thos pages.
It seems almost impossible to analyze all of the implications. Have you thought about removing older content in smaller batches? Maybe get rid of the least visited 5000 pages first to see how that impacts the site overall.
I am not sure I would be comfortable dumping that much content willy nilly without doing some testing.
Another option would be to make sure to redirect the old content to the category page so you don't just lose that link juice all together. I would highly recommend figuring out a redirect plan for that content. Losing the links is a bad idea!