He didn't accomplish the ultimate goal of winning a championship and didn't even make it to the finals so in that sense he couldn't get it done but overall I think his tenure was still a success. I believe the Rockets have the second highest regular season winning % behind only the Spurs since he took over so he was doing something right. They also made a couple of WCF but ran into the Warriors both times.
It also wasn't just the small ball and shooting threes. He helped change the NBA by putting a big emphasis on analytics and making the use of more data widespread across the NBA. He's the one who created the annual analytics conference that important people from every sports league attends. Teams obviously use the data on offense and shoot more threes now but analytics are also used for defense, all of the individual/team tracking, the draft, etc. Morey shouldn't get all of the credit for it but he has played a significant role.
On the last point, I agree. The Rockets are likely going to be a mess going forward. Morey made his share of mistakes and the Gordon extension they're stuck with is definitely one of them. Going completely without a center is probably another one because I don't think that can work full-time. However, I think the two biggest issues they have is their owner and the Westbrook trade and you can't blame him for either one of those. Multiple reliable reporters have said it was Harden who said he wants Chris Paul gone and wants to play with Westbrook. Then the combination of Harden and Fertitta made Morey make that awful trade. There's no way someone who obsesses over analytics like Morey, at times to his detriment, would trade for an inefficient non-shooter like Westbrook by choice.