Curate, connect, and discover
It is not that Shirou and Atsuya are less interesting when they're both alive, It's that Level 5 didn't do them justice at all in Ares (And probably Orion too)
The brothers had the potential to still be interesting. Sure, whatever they did wasn't going to beat the Og story where Atsuya died BUT they could've still come pretty close and made it work just like they made it work for Kira Hiroto.
I plan on making a post with my overall thoughts on Ares so I'll probably be talking more about this topic there.