Nope, base miles have never been debunked, endurance forms the basis of training for all endurance sports. More recent research into polarised training has shown the best way to train is with a very high volume of very low intensity.
There was actually a great interview with our female rowers at the olympics that no-one picked up on. Despite the fact their event is less than 10mins long their training regime was very very predictable – it was founded upon long slow rows day after day. Same applies to cycling, running etc etc.
Where the line becomes more blurred is for ‘time crunched’ athletes, but even then you can’t do nothing but intensity. Look round the internet and you’ll find plenty of stories of people hammering out sweetspot sessions day after day, and in some cases overtraining so badly they hadn’t recovered years later.
Going back to a traditional periodised approach, which i believe kryton is following with some slight tweaks to compensate for low volume – its about a steady build making training more and more race specific.
Taking skills aside, XC races are a blend of muscular endurance and anaerobic endurance, both of which are underpinned by good old fashioned slow mile munching endurance. The bigger the endurance base, the more potential you have to grow the muscular/anaerobic parts closer to spring.
What has been debunked (as there has never been any evidence) is that base should be exclusively low intensity and any intensity in that period will undo that work. Working on other fundamentals like force, pedalling economy and moderate muscular endurance sessions (tempo) during base makes very good sense.