Did you see how hard Elon tried to get out of the Twitter deal? Remember how he made up fake excuses about bots to try and torpedo the merger? Even Elon doesn't think Elon can make the Twitter deal work.
We don't know for sure that's why he tried to get out of the deal. Maybe he decided he actually wanted to focus on Tesla/SpaceX, or was trying to gain leverage for a lower acquisition price.
If he didn't think he could improve on the status quo for Twitter, the thing to do would be to trim as much staff as possible to cut costs and then continue with the current ad revenue based model, without making any changes or introducing new features. Twitter was making on the order of $5B/year in ad revenue. Pushing costs below $5B/year should not be difficult given the nature of the service provided.
Alternatively, if what you say is true and he doesn't think he can make it work, he could try to "back out" by flipping Twitter to another buyer, or just take it public again. Revealed preferences suggest he thinks he can outperform those alternatives.
