The perception is definitely there that they are on the weaker side. We are just out of the Vayne two-shot era, and Varus and Corki still get decent amounts of solo lane play. Think a thorough answer here would take awhile so allow me to piecemeal some of my thoughts at ya.
Statistically, seems like support is the role that's wanted to play the least. This certainly isn't new. But it does impact marksmen, when their partner is unwilling to play along with them. Likewise, if you have people playing less supporty and more magey champions in that position, the marksmen position is going to feel worse. You have 1 champions worth of power, instead of 1.5/2.
I think explorations like Ziggs bottom are awesome. Wish we saw more of that get acclaim. Am definitely unconvinced that it's optimal. I'm super hopeful it's actually decent.
Don't think there was an intention of taking power away from marksmen, necessarily. Couple of things that are semi-new that contribute to their power being down: few of them relied on penetration builds that are worse, junglers tend to be slightly stronger earlier and have more presence bottom, mage supports are more popular, people who kill marksmen are popular (divers from the jungle, Syndra in mid, assassins to a lesser extent.