Domain names are maybe becoming less important to a percentage of the growing number of net users, but the situations where domains are important are not reducing and will still grow from where they are now and have been in the past.
Were less umbrellas being sold after the invention of the car ? No of course not, because it was still a growing market, and as the population increased and more people became more affluent more people could buy umbrellas and just because you own a car doesn't mean you don't own an umbrella anyway.
What would seriously damage the volume of sales for umbrellas would be, if it never rained.