My feelings are mixed. Obviously you could do way worse. But, while I liked Jurassic World, I didn't love it, and just because it's a mega hit doesn't necessarily mean its director should be given this trilogy's conclusion. Trevorrow clearly has a more old fashioned approach than most modern directors, but he's not perfect. JW was enjoyable enough but I thought the pacing was a little off in spots. And he's probably better at directing than screenwriting, so I sort of hope that he's not taking up writing duties.
On the other hand, Rian Johnson is still signed on to write this one and Trevorrow revising the script is sort of to be expected. It probably shouldn't be a surprise that Trevorrow was hired though I guess I was still holding out hope they'd sign on Brad Bird. But maybe this is the next best thing? Bird himself has said that Trevorrow reminds him of himself and the two almost co-directed Episode VII (which should have made this hiring even more obvious).
I guess the most important thing to be happy about it that there are different directors for each film. Not only is this keeping with the OT, it ensures that each film will feel like its own beast and that each film will have its director giving it his everything with the knowledge in mind that this is their one chance to make a good Star Wars movie - so they'd better not mess it up. One thing I notice a lot these days when sequels are announced to films that aren't out yet is that the filmmakers often seem to be too busy setting up for other movies. With different directors, you can be sure that they're not going to want to have they're one shot at the galaxy far, far away be a throw away feature length advert. Similarly, each film will be fresh and, with new blood in for the trilogy capper with their own ideas, hopefully Episode IX will not fall victim to the curse of the threequel (even if I'm already anticipating a repeat of the reaction to the OT - 1 is the audience favorite, 2 is the fan favorite, and 3 is mostly regarded as the least favorite).
Anyway, there's really no reason for anyone to be pessimistic about this for the next four years. Trevorrow is a talented filmmaker and I'm excited to see what he brings to the franchise.
This post has been edited.