Your concern is understandable. Assume you clog a 1" diameter pipe working at full suction (Herbie requirement) you cannot replace that with a 3/4" drain. HOWEVER, (first point) for your main line to glog 100% immediately, some real strange thing needs to happen. Like if someone sticks a pingpong ball in it or something like that. Make sure you have a grill covering the intake of your main drain and you should not have a problem. Typically, your main drain will slow down with time due to stuff sticking to the inside of it. That is expected of any reef plumbing. What happens in this case is you will start noticing a little water coming down the backup drain. This is the main reason why the end of the backup drain should be above the water surface in the sump, so you can hear it dropping water in the sump. At that time, you can try to find the reason why it's doing so or simply open your gate valve on the main drain a little more and call it a day. This brings me to the second important point: You never run your 1" main drain at full drain capacity. If I remember correctly that's something like 2,600 GPH. Unless you run a 5,000 gallon system, that's too much. So assuming a 120Gal tank size and a target drain flow of about 800 GPH, you will dial down (choke) your main drain to that capacity and that full capacity can easily be handled by a 3/4" backup drain.
Summary: No worries, you will be fine.
