I honestly forget about the tunnels every time I'm in Downtown Houston. I've never visited them, and never think to when I'm in town. Maybe I'll do it next time I'm there. Being the postcard collector that I am, I'm sure there are some good places to find some there.
My take on the tunnels possibly kill street life is, so what. Look at this way, the whole point of having density and urban neighborhoods is for the sake of density and sustainable growth and convenience, right? There's undoubtedly density in downtown with residential, hotels, restaurants and offices. The tunnels only move those people off the street into a cooler environment, but it's essentially the same as street level retail above ground. People are still walking, not driving, and it's obviously lively. So I see little problems with it. The only thing I would be concerned about is if that model of built environment ever became less desirable and outdated. Indoor malls these days are on their last leg. Most of them in Austin are either empty or being torn down because of their vacancy rates. I do wonder if that could ever happen with the tunnels in Downtown Houston since they're essentially the same as an indoor mall. Anyway, personally for me, I'd rather be walking at street level enjoying the fresh air, but I suppose if I was doing that and it started to rain, that I might duck for cover in the tunnels.
And anyway, real Texans can take the heat.