Tbh I don't think it is a replacement for the G1, but more like a different class of model. For example you have the HTC touch. No keyboard etc, more for casual users, and the HTC tytan 2 which was more of a business phnone.
Neither was a replacement for the other, but just for a different audience. Like the G1 is for one type of audience (maybe the more business orientated) and the G2 for another type of audience (maybe the more casual user) if you get what I mean.
I imagine the replacement for the G1 will just an up-spec'ed version (i.e. better hardware but still with keyboard etc). While the replacement for the G2 would have better hardware, but no keyboard. TBH, I'm waiting for my ghey orange contract to finish so I can get an android phone. Not sure which to get, but I'll make that decision when it comes to the date.