You said that Riot is forcing Runaan's, which is an item with almost no play. So I asked if that meant you didn't have a problem with how forced Infinity Edge is. An item that is seen literally every game.
I don't think Infinity Edge is a forced item: it's more like necessary
Every Moba showcases at least one item that modifies the basic attacks' strength and considering AD Carries are renowned mainly for their basic attacks than for their skills (we have some exceptions though) and considering that almost every Moba implements the critical chance/dmg mechanic I don't see how the Infinity Edge (in LoL case) can be considered forced
However, Ruunan's case is different: Infinity was made way before Ruunan (almost a 3 years difference), making Ruunan more of a situational item than Infinity. Even though some AD/AP carries can use Ruunan for some increased utility (Varus, Kayle, Jinx), that utility comes mainly because of their own kits (Varus for his W, Jinx for her Q AoE, Kayle for her passive). However, even for these champions (except Kayle, since she's AP based (quite the only one)) getting an Infinity is most of the times better than an item which gives you mainly attack speed: an AD Carry's job is to focus enemies one at the time and for this it is better to have increased dmg + critical than attack speed + 2 more hits, since usually in a fight those who focus one enemy at the time dish out more dmg than those who equally hit all enemies around. Also, other attack speed items are way better than Ruunan, since most of them give critical chance
Said this, now about why I think it is getting forced: taking still in consideration the already-named champions, with Kalista they made a champion whose main source of damage was about putting the more stacks she could on enemies. They basically did a champion that greatly benefit from Ruunan, or better, they did a champ that absolutely needs Ruunan to be played properly. And here is where I think the problem is: they first made the item and then the champion, not the opposite; Infinity basically came out with the first patch ever released (or so I think at least (unless I read wrong)) because it was needed to define the role of the AD Carry; Ruunan is a completely optional item that most of the time is avoided unless you make a champion that has a kit that beneficiates from it
With Ashe, they basically did the same thing they did with Kalista: they gave her an ability which makes Ruunan needed to bring out her full potential (for those who do not know, her Q cannot crit if not only once)
Basically, to summarize my position: Infinity is not a forced item because it has something that goes well with any champion; Ruunan is a forced item because they needed to make champions such that it could be actually used because needed