You have made a valid point. Now let me give a partial answer to my own challenge.
I am very willing to consider the possibility that Islam is the true religion. If this is so, then I will be judged on the Last Day in accordance with how well I had lived in accordance with Islamic principles. Whether I am sent to heaven or to hell depends upon these scenarios:
1. I will go to heaven if it is true that all persons who believe in God and who do good works will be saved. Sure hope so!
2. I will go to hell if it is true that only those persons who have embraced Islam will be saved. Definitely possible.
I would hope to go to the Islamic heaven, because (for one thing) I do not think that I would tolerate being in fire very well. But I would probably be willing to accept either outcome from an objective standpoint.
So I think you can see one reason why I endorse the idea of universal salvation: I don't want to go to hell myself.
There, that was not so hard. Anyone else want to chime in?