Phones and sim cards a lot more temporary than ID cards. I don't know of a lot of theves that target ID cards for their authorization uses. Phones... people will steal those.
You can change it in the app, yes.
> How to issue new key if needed?
I think you’ll have to reissue your ID.
There’s also digi-ID (similar e-signature certificate on a card, but without any ID features), Mobiil-ID (e-signature on a SIM-card, no idea how it works), Smart-ID (in app, tied to secure storage in Android/iOS, cross-signed by the server which is supposed to check the device somehow) and probably something else I don’t remember. All of these are independent options, so you can, for example, revoke your Mobiil-ID if you lose your phone, and still use the your main ID card to sign things.
Is the app tied to Google or Apple?
(Though Smart-ID is its own thing and is a fair bit more locked down, but I’ve managed to get it running on a phone without Google services IIRC.)
But, like the parent said, you have many other options other than the physical ID-card as well. Most people these days use Mobiil-ID or SmartID, which works on your phone and even smart watch. SmartID is completely free and Mobiil-ID is tied to your phones carrier, so the cost varies, but it's a one-time set-up fee of around 5€. Mobiil-ID certificate also lasts 5 years.