whether or not you get a compensatory pick is based on a formula. part of that formula is how many of your players were signed by other teams and for how much, but another key part is how many free agents your team signed and for how much.
in holmes' first year he let mutiple players walk for big deals (golladay in particular) and didnt sign anyone to anything more than a cursory deal. thus, we got lots of comp picks the next year. we havent received many (any?) since because we signed a lot of players to big deals.
if you want the lions to sign a bunch of free agents, they arent going to get many - or any - comp picks, even if they let a lot of flotsam and jetsom leave the team.
if they reach the 49ers or chiefs level of roster where they have to pay certain guys and have no more space, then they can use the comp system to recoup midround picks. but that means losing good players and not signing big name replacements.
the folks who are calling for the lions to sign every 30 year old big name free agent under the sun are the same ones saying we should trade assets because we'll get comp picks back. no you wont. or you wont get a good comp pick. maybe a 5th, mlre likely a 6th or 7th.
you get a 3rd round comp pick when you lose a good player and dont sign anyone else to a big deal.