I run Google Patents (patents.google.com) - we've thought about including a way to search by expired patents (we have the expiration indicators), but in my opinion it gives a false sense of security. There's also http://freeip.mtu.edu/home/index.php, which searches over only expired patents.

The patent in question could have been improved upon, and that improvement can still be in force. Say someone patents a widget A + B, and later files a continuation A + B + C (https://en.wikipedia.org/wiki/Continuing_patent_application). The first patent could be expired, but while building your copy of A + B you might come to the same conclusion that the invention also needs to include C (which is still in force) to actually work.

Google Patents focuses on improving patent quality. There's still uncertainty if a granted patent is actually valid. If we can improve the prior art finding process for inventors and examiners, then fewer overly-broad patents will be granted, and it will be easier to tell if an invention actually infringes a patent.

Then we can start to think about making patent information more useful for part of the original purpose - as a transfer of knowledge to the public domain in exchange for a temporary exclusive right.