It seems they are planning to keep using these planes for some time !
Why not? If it meets the required strategic roles and is efficient and economical, why not?
Iran, unlike the US, does not have a military industrial complex which is bent on making new models so they make more profit. Iran is all about strategy and economics.
If they can spend 1 million dollars and cause as much damage as a 100 million dollar project, but it doesn't have all the fancy gadgetry and sophistication, then that is what they will do.
War, as you know, is all about strategy and not about hardware.