This will become very expensive for a lot of people who use web hosts that do not offer LetsEncrypt, but instead charges a lot of money for SSL. Mine charges $75 per site per year, so with multiple sites, this can get expensive. The host I use, and have for many years, makes me cringe to think I may have to move to a new host with LetsEncrypt, but I may have to make that sacrifice and move.

The fact people will be seeing these warning messages before entering a site, could end up costing a lot of visitors and money to the site owners. People will be scared off when they see the Not Secure messages.