The performance impact of HTTPS is negligible.

Article from 2010: https://www.imperialviolet.org/2010/06/25/overclocking-ssl.h...

It's called "competition". Clients can make sure the ones not too interested become very interested in a few easy steps.

When we are talking about cheap shared hosting providers, the only competition is price and it's driven extremely low.

Competition can only kick in when browsers go crazy about plain text etc.

Well, if the client doesn't care about that warning, why should the provider care?

As I said, there are easy steps clients can use to make their provider care. But if the clients themselves don't care, nobody will.