1. If Yuzuru were to have retired (and when he does), i'd be upset. Not dramatically so, or anything, but athletes like Yuzuru dont come around often, the last skater to win golds back to back since Dick Button in 1952.
The sport would lose a major figure and it would feel different. His retirement would leave a very noticable void for some time as people adjust to the new change that Yuzuru is no longer a main player.
Personally, id take a break from watching skating for a while until the rawness of it all wasnt so bad. I'd still watch skating, but more casually.
2. When he does retire, it depends largely on the fans he brought in with him. How many have fallen in love with the sport to remain? Is there another skater they will choose to follow as actively? Or were they more drawn to Yuzuru? I think for a time revenue may go down, but i dont think it will be too dramatic.
When Evgeni retired i thought it was the end of an era (im a bit dramatic lol). However, Yuzuru rose up to command the stage it seemed. So, sooner or later, i think another skater would take up the mantle and interest would get rekindled.