Don't reject soloing outright. Talk more along the lines of how EQ was (Which I know you've been doing)--say "mobs are so difficult, that grouping is the safest, more efficient way to defeat them. However, particularly skilled adventurers, with well chosen gear and spells, can challenge themselves to overcome them."
See, the Dark Souls crowd are essentially the same people that were solo chanters, shamans and necros+Duo partners (Monks, SKs, Clerics ect). Pushing the boundary of what they could do by memorizing precise timings, patterns and extreme usage of their class abilities. EQ was so "free" about letting people kill high end stuff, that, in fact, on the p1999 EMU this community of solo artists still play this meta game (And they existed on the live realms, too.)---
P99 Solo Artist Challenge. Now, there are benefits to killing these mobs sometimes, but if you notice, for a lot of them? It's just the fact that they are hard.