Roughly 8 in 10 registered voters who watched the debate (81%) say it had no effect on their choice for president, according to a CNN poll of debate watchers conducted by SSRS. Roughly equal shares of Joe Biden and Donald Trump supporters said the debate had changed their mind.
So, it was a wash. Other polls may differ, but I don’t think any debate has mattered in the last 20 years at least. They’re from a time in which people got less information, so people were genuinely undecided and got new information out of debates. Now they’re held because people will watch them.
This isn’t to say things are looking good for democracy, but the debate is not likely to have moved anything.