Not reading the whole thread, but.....
I dont think sales figures have much to do with it.
It's hard to judge how viable Tim is right now because he hasn't had anything close to a solo. He's been saddled with a broken, mostly-dead franchise in the Titans with a writer who isn't very popular, and then he got put into a Bat-book where sales are always solid, with a writer DC has a lot of faith in. Tim's name wasn't on the cover in either case. Was the success (or lack thereof) of those books due in any part to Tim? DC might have market research to answer that question, but we don't.
The other Bat-kids have either recently had solo's, team titles with their names on the cover, or both. That shows their viability far more than what Tim has seen lately.
I suspect the problem DC has with Tim is the same problem they have with Tim's entire generation. The Young Justice kids are no longer the new up-and-comers, since we have Damien and Jon, but they're not the young adults and OG sidekicks that Dick's generation is. DC hasn't seemed to have an idea for what to do with these kids for a long time.
However, it does seem clear that DC has finally figured out what they want to do with the YJ generation, and we've been seeing a slow burn buildup since Rebirth started. We've seen Conner hinted at and mentioned and *a* version of him showed up at one point. Tim played a huge role in 'Tec. And Bart just finally found his way home. So we're getting back to these characters, DC is just dragging their feet about it. What ends up happening with them? I have no idea. But I strongly suspect the lack of the YJ kids (or the utter butchering of their characters) has more to do with DC's creativity than it does with sales.