Decidueye is the most popular of the final evolution starters, though. It doesn't lose any marketability at all.
And claiming that Rowlet will fully evolve because Ash's birds always fully evolve is a weak argument. It's a pattern, and pattern's are meant to be broken. That being said, I think Rowlet will fully evolve for three reasons (two of which are the same reasons I think Lana's Popplio will fully evolve):
1. Rowlet's battle style. From what we've seen in Rowlet's battles, it uses Leafage to hide itself and sneak up on its foes. That just screams "stealthy archer" to me, and I don't think the writers would have given it that specific battle style if the intention wasn't to evolve Rowlet all the way.
2. Like Primarina, Decidueye is a popular and marketable starter. Like I said, it's the most popular of the starter final evolutions. Not evolving Rowlet would be a huge missed opportunity.
3. Again, like with Primarina, they have to show off Decidueye in the anime somehow. If Ash isn't going to be on the to show off Decidueye, I would love to know how some of the people that feel Rowlet won't fully evolve think the writers are going to show off Decidueye. Throwing it into a PotD filler is unlikely, because Decidueye seems too popular for that, and it has a unique Z-Move which can't be shown off in a PotD filler. Anime OC rival debuting it is possible, but given the structure of the series, I have a feeling that Gladion is going to remain Ash's only rival (Island Challenges aren't like traditional Leagues, I highly doubt there are that many people running around taking them). I suppose it's also possible Gladion could randomly have a Decidueye (it's definitely a Pokemon that fits him), but I have a feeling that if he did have one, we'd be seeing it with his Lycanroc and Umbreon in the next episode.