Grinding is a part of online games. Every online game has it. And you know what? Its fun.
You might disagree with the typical "hur dur is boring so i dont do it". That is just a childish vision. Much like a child vision of work.
When you grind, you are putting in hours and work to get what you want. I put in several hours, at least 5 straight hours in Violent Duct and it felt good after. Because I felt like I had earned my money.
However, the bug where you can bomb the coke/bomb the documents in rooftop rumble made it so ridiculousy easy that even I agree it should be removed.
Once removed, these missions would still take a minute or two. It would depend on your skill as a team to complete it fast.
And it would be incredibly fun.
TL;DR: Let us replay missions, but remove the destroying coke/document glitch.
You just answered yourself why they want to lessen the farming. Because you didn't enjoy yourself during, not until after when you looked at how much money you had meaning you were really only doing it because it was easy money, even if you have the conscience to not exploit glitches to get it.
The fact of the matter is people are acquiring money and rp much faster than they intended and at a rate that it will be impossible to be anywhere near keeping up with content wise.