I don't doubt that his decision to stay aboard Enterprise as her XO was seen as something of a surprise by the brass back at Starfleet command.
It was a let down for me as well.
I always wanted Riker to be Captain, finally got my wish.
Worf agreed with me as he requested to transfer to Riker's new ship if he took the promotion, as I recall.
Even if it had meant his leaving the series, I knew someone would immediately start writing novels of Riker's adventures, as they now do.