> We don't know that electrons don't decay for sure.

However, we don't expect electrons to decay as we don't know what they would decay into i.e. there doesn't seem to be anything plausible with a lower energy configuration.

> If we live in a false vacuum, for example, that could allow them to decay.

Possibly, but that's quite speculative and if our vacuum does decay, then there's a good chance we wouldn't be around to see the differences.