Well sure, he's a decent Director, and I think he's a good actor. But I just don't think he'd have given TNG the gravitas in a lead role that Stewart brought to bear. I mean, I think most of the TNG cast are talented actors...Jonathan Frakes and the 100+ episodes of television (and several movies) that he's directed would like a word.
It's funny because in the original concept for TNG, Picard is supposed to be kind of the second lead. You know, Riker was the one who went on the missions while the Captain stayed on the ship. But as time went by, Picard came to dominate the series along with Data and to a lesser extent, Worf. And I think that's because those actors were the best ones.
It's a fun thought experiment to imagine if they brought on Avery Brooks or Kate Mulrew or Richard Dean Anderson after BOBW. Would it have worked? Maybe. But we'd loose those incredible tour de force performances that Sir Pat put in.