Not Marketo related as much, but do you have a sense of the necessity to have LPs secure? My thinking is that HTTPS is helpful for search, but our Resource landing pages don't rank (at least highly). They are typically accessed from internal links from our site or blog posts (which is secure) or an email link. I'm trying to understand the impact if we leave our Marketo LPs as-is.
Well, as in the thread topic, there's the cosmetic concern that pages will soon say "Not Secure" up in the left-hand corner. The impact on conversions is unknown at this time. 
Agreed that if you're not worrying about search, then Google's preference is a red herring.
I look at things (as probably know from reading my rants here) from a risk and security perspective that few in martech share. It depends on the type of company you work for, but when I look at how easy it would be to hijack an insecure link and send it to a phishing domain, it's impossible for me not to see that as A Bad Thing.
Also, on an analytics level, if anyone links to your LPs from a secure page, unless they take special measures (few do) you won't see the referrer, since that's not sent from https to http pages by default. But if this almost never happens with your LPs it may not be a big deal (though sometimes you want to know that one blog that loves you!).
I would categorize the SSL decision as "Better safe than sorry" (even when it comes to the Chrome warning, surely it would be better to not have to worry about it) but Marketo's pricing is so high it can make you reconsider what should be a no-brainer. That's why I recommend using a CDN until they get their act together on that.
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