Your dbFront license is perpetual and will not expire. You will also continue to have access to the website and the Q&A and you will be able to ask questions on the Q&A site.
Without a Support & Maintenance agreement, you will lose access to live and priority technical support and you won't have access to install upgrades. Newer versions of dbFront often require an updated licence code.
If you look at dbFront's Release History you can see that dbFront is under active development and we are adding new features all of the time.
If you skip out on the Support & Maintenance then you will miss out on any new features.
If you avoid paying Support & Maintenance for 2 years and then decide you want to upgrade then you would need to purchase a completely new dbFront license. After two years there is no upgrade discount.
For more details see: Support & Maintenance