So not only the ads collect personal data, slow down web experience and make every non-adblocked site a pain to watch... you go the extra step to not even encrypt transfer due to them?
And anyways you can solve it in the same way: nginx LBs to terminate SSL internally.
Also, if you have an ad network that can't serve https, just stick another reverse proxy like HAproxy in place and convert their http content into https for your clients. It works very easily for that type of thing as well - in fact, you can even do path-based routing like https://www.mysite.com/ad/* goes to http://ad-network.com (while https://www.mysite.com goes to your regular back-end), and HAproxy will do all the rewriting for you.
Also, most ads are now html5 with many resources and scripts etc. for interactivity. You can't just host it on another domain - there will likley be lots of internal absolute paths and paths assembled with JavaScript that one can't practically rewrite.
Remember most sites use an ad-exchange, so we aren't talking fixing up one or two ads here - there are probably over 1 billion unique creatives to fix up.
It's an established standard and now completely free to set up (thanks to Let's Encrypt ;)). I don't see how these two (yes, frankly, two reasons. let's really ignore the third one) can keep anyone from providing HTTPS connections to their dear readers and users.
To be fair, I would expect any reasonably large company to obtain an EV (Extended Verification) certificate, which is very much not free. Let's Encrypt only does DV (Domain Verification). EV includes verification that you're actually the incorporated company that you're telling everyone you are.
If you use permanent forwarding (HTTP 301) then you should be fine.
I believe Google says they use HTTPS as yet another signal they use to rank your site, not that they will significantly push up HTTPS sites over other signals they use, like 'quality of content'.
1. a lot of third party advertisers/ad servers still run on http, these ads need to be embedded via DFP usually, which you can understand does not work out well. We made the switch months ago, to this date i am still making advertisers switch to https.
2. google says they give better ranking to https sites, thats simply not true so far as i have seen. in fact, in the short run your site takes a hit. not only that, in google webmaster console and google news, you cant shift from http to https, you have to make new accounts for ur https sites. to this day i do not know which ones is google crawling. For google news, my new https account has yet to be approved after months, it looks like google just magically shifts to https in google news. but if feels icky and hacky: explicit is always better than implicit.
3. microservices. remember those microservises that were all the rage? well, its a bunch of different servers and subdomains, you have to shift all to https when you shift the mothership to https.
while above points are valid, i still pushed in my org to shift to https. we now use shiny stuff like http2 and web push, which is awesome. i'd recommend all publishers to do so. but its understandable that management finds all this scary, esp cuz its sounds like a major overhaul of your web assets (which is everything when ure a web publisher) - even though it isnt really actually an overhaul or anything.