GLP-1s aren’t “forever drugs.” They’re eliminated so fast from your system you need to keep taking them to keep the effect. That’s the opposite of a forever drug. If you go off them, you just return to the way you were.
Which means if you want to maintain their effect, you need to keep taking them forever. So maybe GP used the wrong term but I suspect they meant exactly this.
In any case, we just don't know what kind of long-term effects these drugs have. No one knows what happens if you come out of them 30 years after you started.
I think they mean they're drugs you have to keep taking forever (for the rest of your life) to keep the effects.