There are other options besides "doom and gloom" and self-indulgent silliness. Of course, it's challenging to make a broadcast interesting when the team isn't, but unless you're lucky enough to work for a perennial contender, it's part of a baseball announcer's job.
This year, Len and DJ are doing pretty well with the radio broadcasts. I've heard them call guys out for bad plays, get mildly excited for good ones, and spend a lot of time talking about more interesting things - like other games around the league, good players on other teams, baseball history, insider strategy, etc.
I think one of Schriffen's problems is that he doesn't seem to bring much to the broadcasts other than the silliness and some notes from the previous day's post-game interviews. He relies a lot on repeating press conference pablum: "Pedro says Andrew is starting to see the ball better," "Jordan is happy to be back in the big leagues," etc. You might get away with that if you're calling a couple games a month for ESPN, but it's not enough to carry 160 broadcasts a year.
He lacks Benetti's connection to Chicago, lacks Kasper's knowledge of baseball history, can't spin old yarns like Hawk, and doesn't seem to have any special access or insight into what's going on with the team. The silliness and funny noises will appeal to some viewers, but he'll need more than that to endear himself to a bigger audience.