Euston Square station is probably closer to Warren Street than it is to Euston, or at least that what it looks like at street level.
I've been using Warren Street station for a good number of journeys recently and I think from an infrastructural point of view, you're right, but pax will associate Euston Square with Euston and think they're closer, which is how it feels at street level anyway.
It would be quite costly, but having a subterranean interchange between Euston Square, Warren Street and Euston itself would be useful in the event of disruption and would probably involve passages of a reasonable and certainly not unheard-of length, and I am pretty sure, from what I can see from tube trains, that there are quite a few staff passages of a similar nature anyway.