Since this is back in the news, let me offer an argument as to why I’ve never thought this was a good idea:
1. It's not the most important issue
It's what SOME people think is the most important issue. Polling indicates that other issues have greater resonance with voters; some (myself included) think health care is most important; others think immigration. DNC shouldn't be calling out a single issue as deserving of special coverage.
Now, some people will say "why not have a series of issue debates?" Beyond that fact that DNC is then picking and choosing what's important to discuss, this leads to.....
2. Saturation will dilute the debate audience
Either you're taking away open-issue debates to accommodate climate change and other issues, or you're adding events to the schedule. The more debates, the less likely people will watch them, especially if they're issue specific.
3. It won't inform the audience
Every candidate in the Democratic field agrees that climate change exists and is a problem that has to be dealt with. The only real dispute is whether to implement "Green New Deal" or something more incremental. Those answers can be found on each candidate's website; having 5 minutes each to "debate" this point isn't going to provide a lot of information for voters to consider.