The keyword here is a maxed LB 4* is stronger than a 5* +2. However, a 5* with LB easily outclass 4*. When you get to higher rank arena, you can't rely on your winry to deal reliable damage. I'm not going to waste resource on a 4* that won't be useful in higher arena.
The only thing that would "open my mind" is when someone within the top 200 use a 4* heroes in their arena team.
if you have a 5* unit around you should of course upgrade them. if you don't then there is not much of a resource waste as 4* can be easily trained with 1-2* units from the gold gacha. actually if you do a bit of research in the gallery, a max trained (not lb) 4* is only very slightly weaker than a fully trained 5*.