Did Trump win or lose by leaving the debate stage?
Lots of people are saying the telecast was a Big Success without Trump. Other people are saying that they lost half their viewers compared to the first Republican debate this year. Who's right? Well, technically both claims are true; it's a question of which spin the speaker is trying to put on the information. Let's look at the actual context. For the purposes of this post I'm going to trust the Wikipedia numbers:
OK, so the first debate numbers to the last debate numbers did drop by almost half. But they did that over the course of 7 debates. The big drop occurred after the 2nd debate (9 million fewer people tuned in for the 3rd debate than the 2nd debate). Debate 5 was an outlier that jumped back up; 6 and 7 were consistent with most of the previous results. If anything, Trump's absence slightly reversed a downwards trend. We can only guess at causation for those numbers, of course, but viewer count was trending mostly down with a small blip up for the last debate (and a much larger blip up for debate 5).
So, if your measure of winning or losing is how many people watched compared to previous debates, or the trend of people watching with or without Trump, Trump lost. About the same number of people showed up as had been showing up before. And maybe a few more showed up because Trump wasn't there. Maybe.
I'll note that a declining trend is common for situations like this. People lose interest in watching the same group of (frankly, mostly boring) people say basically the same things on basically the same topics over and over. For most people, you watch until you make up your mind, but only political junkies keep tuning in for 6 or 7 debates (remember, that's probably 20-30 hours of programming if you count the mini-dates too).
So the declining interest is normal, and in context with the other debate numbers, removing Trump from the debates didn't move the needle much.
I don't have numbers for Trump's competing event in front of me. I've seen reports that he did not match or beat the debate numbers, but (right now) I have no idea if it was by a lot or a little or just not yet confirmed. We'll see, or I'll find numbers before I post...