We already got enough problems with catching up to certain Mages and high priority targets and most of the frustration with Rylais comes from how easily Mages can keep themselves out of harms way simply because Rylais allows them to become kiting beasts.
Viktor, Azir and Co. with their high range and low CD's are especially frustrating to play against because they can simply apply the slow from range.
Hell, the bit of health it provides isn't really an issue as most champions who can close the distance have more than enough damage to kill them but the kite is really hard to deal with.
Not to mention most of the champions who "abuse" Rylais have other CC, often even hard ones to keep their assailants at bay.
Most ADC's are toast if they suck at kiting, and they manage to do so without a perma-slow item, so why do squishy mages with enough tools in their kit need to have even more safety?
Rylais should be an item option to combat kiting, not the other way around.
A lot of its intended users (Singed, Mordekaiser, Karthus, etc.) are champions who rely on its slow to even get their damage off and deal with getting kited to hell and back due to them lacking dashes or long ranged hard cc.
I'm tempted to say that mages building it is a problem because they become way too hard to escape from/chase/engage on due to them just kiting you.
Nerfing its damage and the slow amount on maybe ranged/aoe/whatever would be the best option in my opinion as it would make it a ton less desirable for most mages who just love having a high damage item that provides them with all the safety they need.
Make it somewhat like Zhonyas as in being an OPTION, cheaper and usually being built either by champions who just need it or as a situational item if facing a lot of slippery teams.
But really, I feel it would be overkill to give "Squishy Mages" an even bigger safety net seeing that they already have the tools to stay out of harms way and them becoming virtually inescapable/untouchable would be really annoying for a lot of champions.