Before I begin, I'd like to point out that I'm one of those dinosaurs that uses IMAP clients because I don't like my UI held hostage by browser renders. I also like being able to archive offline, prefer radically more customization than browser-based email can give me, and prefer the bulk efficiency of a dedicated program to a browser plugin.
That said, UI is important.
Most people who use Gmail basically understand 10-20% of it. They don't use filters, they don't use autoforward, they don't beta-test, they don't inspect, they type email and send it and if they need something, they don't search, they scroll down until they see something familiar. Most people who use Gmail have an endless sea of icons on their desktops, nineteen pages of apps on their phones and know how to run their digital lives the same way they know the cinnamon is next to the allspice because they smell similar.
Most people assume that when Google gives them a new button, they should use the new button. They presume that Google wouldn't bite them in the ass on April 1 because Google has never bitten them in the ass on April 1. Most people have a hard time attaching .gifs to their email to begin with so the idea that Google would append something that might destroy their conversation is not something they're likely to consider even if it's spelled out to them.
Most people will click on this link if they think it comes from you:
Not because they don't distrust the URL enough, but because they trust you too much. Scams don't work without social pressure and when your very client gives you something shiny and new, you're going to use it unless you're
C) paying attention
D) in full command of the consequences
If Google did an April Fool's joke that read like a Nigerian Prince scam, they could add up a tally of all the money they'd "earned" just to show the world how simple-minded and inattentive most computer users truly are. I mean, yeah - when you explain it as "why on earth would you attach a mic-dropping minion to a funeral arrangement email?" it sounds ridiculous. But when you explain it as "why wouldn't you click on an 'augmented send' button in Gmail?" it sounds just as ridiculous the other way.