The producers/director team seemed eager to make more sequels, but that tends to only happen when enough audience members buy tickets in theaters to see it, which didn't happen in the numbers this film needed to get the green light for the sequel I'd guess.
Keeping costs down for new films in theaters these days can be a big benefit for the studio, I agree. It's been proven many times now, that celebrity star voices are not what get people to see animated CGI movies in theaters. It's the content, story and marketing that does all the work IMO. One example would be the movie Delgo. It had a big all-star voice cast of famous celebrities, and was one of the lowest grossing theatrical animated movies of all time, earning under $1 million dollars with a $40m budget back in 2006.
If a popular voice cast drew in audiences, it would be worth spending the extra money to cast them, but I agree, it's not worth it. Just get professional voice actors who aren't getting over-paid to do the roles.
The trailers were not very good and mostly hinted at trying to be funny, but the movie was generally good, and the 3D mostly a range of medium 3D layers, with one brief scene with strong 3D during the Hologram scene with all the tiny particles forming characters. After that, it went back to medium layers even during action. Age of Extinction Transformers has much more strong 3D in that film.
Although the box office wasn't tripled comparing to the production costs it almost doubled it. That's a good indicator for an OK movie to me.
The sequels might still come and be more successful than this iteration if well done.