I have over 30K miles since late 2012, much of which has been on the SC highways. Napa to Tucson several times, Napa to Santa Barbara, Napa to Vancouver, BC. The only problem I've ever encountered has been at Harris Ranch. Had issues shortly after opening in 2013 as well as currently. Consequently I wouldn't worry about SC's being unavailable. If Harris Ranch is in your plans, you might want to divert to 101 and miss it altogether.What are some viable backup options people have used when planning for road trips that require SC usage being unavailable? ICE'd? Crowded? etc...
As a faithful reader of TMC I don't recall any other instance entire SC's being unavailable. I'm sure someone out there will correct that observation if incorrect.