If he hadn't released all the information proving that what he was saying was true, we'd be hearing about how he was just a disgruntled employee with personal issues who was making stuff up for the publicity so he could sell books, etc.
That's a possible theory. But then why wasn't that the case for Drake? And wouldn't it have been possible to do that with limited data release rather than as much as he did? For example, how is releasing lists of foreign cyber targets, or foreign embassy surveillance useful to he topic of domestic surveillance? It seems they released several thousand documents to prove his point, and yet it's believed that there are over 1M more that he exfiltrated. It seems like that's overkill for just proving you're telling the truth about domestic surveillance. This might have been a case where less is more - if he had taken/released just enough and limited the scope, then there would be more support for him.