I will have no hate for Trent if he leaves... he's been a good player for the club, but people leave. I'll be annoyed we didn't get anything for him and he left on a free, but I won't have any hate for him (as above, I didn't dislike Owen when he left for Madrid, it wasn't until later that I did when he screwed us twice more).
As for legend though... I don't think I'd call him a club legend, a good (nearly great) player, but doesn't quite hit the legend level for me. Nothing to do with being local: Gerrard was a local lad and I think everyone would call him a legend (even if he's also a bellend for going to Saudi Arabia). There are at least three players in the squad better than Trent. Salah, VVD, and Alison. This season I'd say his performance as a whole has been on average below: Konate, Diaz, Jota, Gravenberch, Kelleher, etc...
You can't be at the peak playing age, the 7th best player at the club AND still be a legend. (yeah, sure, previous seasons he's probably managed 4th best... but still, you can't be merely 4th best and a legend).