You can certainly continue using TG3 after upgrading if you wish. However you lose the right to sell the license once you upgrade.
As for monthly maintenance, we are considering subscription options, but they would not be perpetual licenses (nor would they replace the availability of perpetual licenses). Once the subscription expired, you would not be able to use the software. In other words you'd have to maintain an active subscription to use Terragen. This contrasts with the current Maintenance model where the license works forever and the fee you pay just gives you access to updates and priority support. The Substance Live model is interesting in that it offers a path to perpetual licenses combined with subscription. We'll take that into consideration as well.
We have no plans to discontinue the availability of perpetual licenses, subscriptions would just be an additional option for those who wanted the flexibility.
- Oshyan