Concern about a device failure shortly after the subscription is purchased and installed.

A personal Microsoft 365 subscription is tied to the Microsoft account, not to a specific device. If a device fails and is replaced, the subscription recognizes the new device when the user:

1. Sets up the new device.
2. Installs the Microsoft 365 apps while signed in with the same Microsoft account that owns the subscription.
3. Signs in to the apps with that same account so they can activate.

Internet access is required on the new device for initial installation, activation, and subscription management. As long as the subscription is active and the sign-in uses the same Microsoft account, the new device will be licensed correctly, regardless of the old device failure.

If the subscription expires or is not renewed, the apps on any device (old or new) will eventually go into reduced functionality mode until the subscription is renewed.

References:

• What's the difference between a paid Microsoft 365 subscription and the free web apps?
• A subscription notice appears when I open a Microsoft 365 application
• I have a subscription to Office 365 and want to install it on my new laptop - Microsoft Q&A
• move microsoft 360 to new computer - Microsoft Q&A