That argument only works if you’re expecting Google to move youtube.com to youtube.google, which I can’t see happening. If a brand’s a household name and can be found at brand.com, then it stands to reason that they’d leave it like that.

For Google/Microsoft budgets, domain name registration is irrelevant as a cost. Besides, even if they did move the domains, they’d still keep the old ones alive for forwarding and to stop anyone else taking them. For example, Google still has googleplus.com, despite that that was never the official address (they used a subdomain: plus.google.com).