That would be way too complicated for the average user who wants their gifs to play. Given what you want is atypical, isn't it reasonable to expect you to configure it manually? Why should everyone else have to manually enable gifs when that is by far the more common preference?
Is it? Where are you guys getting your stats in this?
Personally I feel like gifs are annoying and distracting.
Come on, be realistic. Tumblr didn't become a billion dollar business just by having good SEO. Lots of people enjoy scrolling through pages of gifs and they won't be satisfied with click-to-play on 100 different 5 second long animations.
Ok, but that's not the point though? The assumption is that The Average User wants these soundless video gif-likes to autoplay. But The Average User never wanted popups. If you don't want them to autoplay, there probably is a setting or extension for it.