If someone gains access to your device, you have bigger problems.A VPN adds nothing on public WiFi. It encrypts communications in and out of the device. So does browsing HTTPS websites. Messaging apps use TLS end to end. If someone hacks the public WiFi and gains access to your device, a VPN won't help. It hides your location, and hides your DNS queries.
A VPN on public wifi provides an additional layer of privacy and security against MitM attacks from bad actors on the same network or even the network provider and their ISP themselves who may (perfectly legally) exploit your browsing history or location data for profit.