Of course, it is technically possible to secure your entire network. That would theoretically eliminate the need for encrypted SNMP. Unfortunately, that level of security is difficult to achieve.
Furthermore, most organizations who care about security would prefer to also encrypt SNMP in order to have dual-redundant security layers.
What you need, in order to give your older SNMP gear the advantage of modern security, is a fairly simple box that collects unsecured SNMP traps before they ever leave the building's local network. The collected traps would then be converted to SNMPv3 before being forwarded to your SNMP manager.
This is becoming the favored solution for military and other government organizations, as well as security-conscious private companies. In today's environment, they most likely don't have the funding to replace all of the installed SNMP gear they've deployed in the last decade. Since they can't afford to compromise security, either, they're frequently using SNMPv3 converters instead. A single converter at a site can handle just about any number of local SNMP (v1 or v2c) devices, so purchasing costs are dramatically reduced.
You can also get more than just SNMP conversion out of an SNMPv3 converter box. Some are also designed to function as alarm-monitoring RTUs (discrete inputs, analog inputs, temperature sensors, control relay outputs). If, like most people, you need some general-purpose monitoring at any of your locations that have SNMP gear, you can now solve two problems with one device.
The key is to choose a manufacturer that will help you to build an effective system that addresses several of your needs at the same time.