I think Lucas lost momentum by waiting so long between trilogies. But to be honest, I think the only way any more movies were going to happen was if Lucas removed himself. I think if he would have tried to the the sequels that he would have ended up with the same problem as the prequels - no one wanted to direct them with him staring over their shoulder. I think we will find that the sequels are really his story with some changes to make them work better. There are hints at that already. They may have moved as far from George’s treatments as some of his films did from treatment to film.
I think they should make some movies, take a break, make some more, take a break. I don’t think they gave Solo enough time. They should have stuck with the Christmas releases.
Trilogies make nice longer stories in digestable installments. Lots of franchises use trilogies. But the one off stories are awesome as well.
I believe Dave Filoni had some trouble making The Clone Wars when it first debuted, due To George Lucas interfering. The Clone Wars movie that came out in theaters is just four episodes of the show mashed together, because they were being rushed by George because he wanted a movie in theaters.
As for the new era of Star Wars, I think that Lucasfilm should take at least a nice five year break after Episode IX before doing ANY new movies (both saga and spin-off films).