I’ve seen a lot of things about this all saying that it’s a fantastic idea. Yes there are complaints about worse performance due to the complexity, cost of certificates, etc. but there is an extremely important reason why you should not redirect HTTP to HTTPS: It gives a false sense of trust.
Imagine a user, Alice, who connects to
http://example.com. Alice has been here before and is already logged in through the magic of cookies. However, when she goes to the site, it redirects her to
https://example.com, but too late. Her cookies have already been sent through plain text on the first request.
A different attack is an active man in the middle attack where the attacker can watch for HTTP traffic and redirects to HTTPS and spoof these websites using similar looking urls. An example of this was presented back in 2009 at Black Hat DC: http://www.thoughtcrime.org/software/sslstrip/
Fortunately there is a method to do this correctly. The HSTS protocol ensures that traffic only communicates with a server using HTTPS. Generally this would mean that you have to make at least one HTTP connection to the site, which is still vulnerable to an MITM attack, however browsers such as Chrome and Firefox keep and curate lists of websites with this flag set so that you never have to risk connecting through anything other than HTTPS.