And I disagree with you :)
Modern era aircraft would take much longer to develop, simply by all the avionics and systems you would need to simulate.
With a less complex AC, they use shorter time, and it generates revenue.
Also remember all the other stuff they work on. EDGE, DCS World, network improvement, bug fixing on existing modules.
Just because a certain module is out of beta doesn't mean they have the resources available at that time to start on a full fledge high fidelity fast mover. So in that case it's a clever move to work on a less complex aircraft until the necessary resources are available.