Well, I'd be upset if I couldn't go too, but I think the point is that the gifts will be needed when the baby is born and waiting until afterwards to give them, defeats the purpose of getting what you need ahead of time. If it's a matter of losing a deposit, or guests coming from out of town, I don't see a problem with having something somewhat themed to the original purpose of the party and in that case, I think the GOH should be a bit more understanding.
At the same time, I think that the feelings of the GOH should take precedence where feasible. If the GOH is adamant that the shower not be held unless she can personally be there, then the organizer needs to respect those wishes. But this barring any loss in money, travel changes, etc. And I agree that while I think most people would organize something else in it's place, there is really no obligation to.
A good friend of mine was on bed rest in the hospital during her pre-planned shower. The shower went on, she was skyped in and she said that while she was sad that it wasn't how she planned to be there, she was still touched that the shower occured. A couple of us went to visit her in the hospital and brought her some of the food and a few of the smaller gifts and then we organized a welcome home mom and baby party later (several weeks later when she was up to it). Being upset because something we were looking forward to doesn't work out is understandable. But I don't think it's understandable to insist that anyone else lose money or have to change plans at a personal cost to them because of it...especially when they are doing something nice for you.