Alex Shibutani, Madison Hubbell and Zak Donohue were all born in 1991, which would make them 31 in 2022. I doubt any of them would consider sticking around for another Olympic cycle. Alex Shibutani has asthma, which has made competition difficult for him at times, so I don't think he would continue for even one season past 2018, regardless of Maia's youth. And Madison Hubbell has had to have hip surgery, so she may not want to further stress her body beyond 2018.
You are right my math was slightly wrong but the difference between 30 and 31 isn't anything to change my opinion- it's still an age at which an ice dancer, if they so choose, could very well be ice dancing, again barring injury... it is the only skating discipline where it is extremely common to peak late. I will admit 31 (or 30 for that matter) is on the later side of late even for a dancer, but it isn't like these are single skaters, it definitely is entirely possible to be dancing at that age, and I could name several who have. Unlike other disciplines, moreover, it's not uncommon for teams not to even really hit their stride till their mid 20's.
As for the health issues you mention, I suppose that's up to the skaters, who are in a way better position than you or I to know the impact these conditions really have on them on average, as well as the impact the conditions do or do not have on their will to compete. I don't see how anyone, myself included, could claim to have any idea whether asthma or a former hip injury impacts an individual's motivation to continue competing in general.
To be clear, I'm not saying these teams will continue beyond 2018 or that they should... as I initially said I think this will depend partly on the results in 2018 and of course on what the couples want to do. I am just saying that they
could continue, and I don't assume like some people seem to that they won't. It's all very well to be like Alex's asthma bothers him he won't continue but unless you know Alex or have read an interview with him saying that, this is just speculation or possibly even wishful thinking.
Finally, I don't disagree with Ice Dance's idea that this doesn't necessarily mean McN and C are doomed anyway- they certainly can muscle their way through, it's hard but other teams have done it in the past and certainly they are talented. I am just saying that I personally wouldn't count on a huge wave of 2018 retirements making it easier for them to do so- if they are indeed waiting in some way under the assumption that the top teams are all or mostly going to retire, they could very well be mistaken in this.