Shame to hear. But at least we'll still get one more season. Hopefully they'll go all out with it. I think the show just never took off like intended. Disney as a company and the actual people creating the show probably had certain great ideas and plans for how the show would be, but overall it ended up being kind of a mess throughout its whole lifespan so far. I feel like it kinda took a dip because, for one reason or another, they never could make the show what they wanted it to be, and then everybody involved sorta lost their interest in even working on it. This is the impression I got anyway. Then the show just never got the chance to bounce back up.
I think there *might* be a chance for Disney planning for the franchise's future a bit ahead, though. The Lion King franchise has been quite dead throughout its entire existence, honestly, and I think this show might have been a good "testing the waters" and "learning from mistakes" kinda thing for them to bring something new TLK related back soon. I don't think they're gonna just drop TLK and forget about it again like they did for a long time after TLK3. They've been doing a lot of shows recently based around their movies, and TLK is one of their most successful/famous brands. Once TLG ends, I think that's it for the series, and it won't be renewed as it is. But I think what we MAYBE could expect from them is that they end The Lion Guard properly, and then sometime after the live-action movie comes out and is fresh on people's minds (assuming it does well) the show's staff will move to work on a new Lion King related show. Perhaps somehow tied to TLG also, but they might decide to avoid making the same mistake and make it more mature this time. Of course, that's pure speculation on my part, but I could see that happening. Funny enough someone mentioned MLP in this thread. Hasbro is currently planning to release G5 of MLP in 2020, and apparently its target audience is gonna be teenagers and young adults. I could very easily see Disney trying to compete by releasing a new Lion King show (a new generation of Lion Guard, if you will), that will be more mature as well. That's what I would do anyway; end TLG properly and tie all the loose ends with TLK2, then after a couple of years with the live-action movie keeping the franchise hot, start a new, more mature series from scratch that will continue the story, but after the events of TLK2.