The most recent advances in web apps bridge that gap! Progressive Web Apps are essentially websites with offline content caching, push notification support and the ability to add 'apps' to the home screen so they behave the same way as a native app, all without needing to be installed.
It gets a bit technical but there's a good overview on the Google developer site here
I'd say this is something to ask your web/app developers about if you're considering an event app as depending on the functionality, an event website and app (which will work on iPhone and Android as it's just a website) could potentially be rolled into one project.
There's a vaguely relevant case study here
And the technology is already mature, the Twitter mobile site is already one of these progressive web apps, although they don't use the add to home screen functionality as they've obviously spent considerable time and resource developing native apps.