There's a reasonable justification there for keeping Riker around for a while longer. We might assume that somebody at Statfleet took him aside and said, "Y'know, Will, you're a captain now. And we won't blame you if you tell us to blow it out our ass. But we'd really like you to stay as Enterprise XO for a while longer and just make sure Picard can still do the job." And then had him grapple with that for a little bit. I mean, 'First Contact' establishes pretty clearly that Starfleet have still got doubts about Picard and the Borg. It would explain why Riker stayed longer anyway.
Meh. I never put much stock in Hayes' decision to keep the Enterprise
on the sidelines. He was an idiot (if we're to believe Janeway's description of him) who liked to hear himself talk. In reality, the writers needed an excuse to have the Enterprise
come charging to the rescue, and Picard's history with the Borg provided them with a perfect scenario for such.