I think it would go against the nature of the show to have Riker leave. Not even Data had drastic changes and was basically his same self all the way up to All Good Things. Sure Picard getting assimilated had some follow up mentions, but for the most part you could watch a season 6-7 episode and never even know it happened.
TNG is a show with relatively static characters I don't expect Riker to really change any more than Troi or Picard did over the years. He's just part of the team and so I find him staying with the team perfectly in tune with the nature of TNG.
The nature of the show didn't have to be static. It was made static by the writers and producers, particularly Roddenberry in the first season. Frankly, television had already evolved in the 80s beyond static storytelling where the toys are all neatly placed back in their proper places on the shelf.
"Hill Street Blues" airing at the same time TNG premiered had shown that you could have continuing story and evolving characters. But TNG was very much stuck in the storytelling style of the 1960s-early 80s. It's a shame the TNG producers/writers didn't look to "Hill Street Blues" as a model. Hill Street was good at handling a large cast and rotating them through stories so they all got a chance to shine. It was also good about evolving them as people and having them affected long-term by the events in previous episodes.