Even if there are zero effects other than the advertised weight reduction, one downside is obvious from the comments here - some people will stop exercising.

While exercise has many positive effects, which I'd argue are more important than the weight loss, the latter is the primary reason many people exercise and likely to stop when given the weight loss by other means.

Nothing in this world is free, GLP-1s have their downsides (excluding cost).

Mostly it's nausea and gastrointestinal distress side effects. These tend to cool down over time, but it'll put a ceiling on how big a dose you can tolerate. Some people can't tolerate a dose at any level.

Don't they age your face pretty badly?

Why does there have to be a downside?

What was the downside of washing hands before surgery?

The downside is that they make you eat less thereby taking away one of the few things that make life worth living.

And downsides of penicilin? There are some but they are massively offset by not having alt consequences