I have a theory I came up with recently while playing PD2. The in-universe lore for both how the MCPD/FBI are able to replace their heavy losses AND why captured enemies can be converted so easily (if you have the skills) They're forcibly conscripting people. They're treating the Payday Gang not like criminals, but as a enemy in a war. That's why the police are much more militarized with units like the Cloaker and Bulldozer and aggressive than their IRL counterparts. They fight like a military because that's how they're being drilled. And like any military that looses hundred of soldiers every battle will eventually do, they'll start drafting in everyone they can.
Converted enemies can be converted because they didn't choose to become cops, they were forced to. They'd willingly thrown in their lot with the winning team.
What do you guys think?