This is more than your 2 cents. This is it. I think it is exactly what everyone think and feel about the stickers marketplace. Big hype, but nothing to support it. You are the proof, with all your efforts, this is the case.
As you said people are way more interested by emojis or keyboard replacements then stickers, mainly because around the world others instant message apps are used and the young crowd use also other means of instant communication (i.e: SnapChat). iMessage is just one in the bucket and If you can find free stickers from well known brands you like or support, why paying for some?
Sad, but all true.