On the one hand, in regards to losing a piece for offense I agree with you but, comparing Herbert's production last season to Monty's (with the same oline blocking) its clear that Monty isn't the feature back anymore.
Monty: 201 rushes for 801 yards (4.0 avg) 5 TDs
Herbert: 129 rushes for 731 yards (5.7 avg) 4 TDs
Another thing to factor in is that Monty's overall production (total yards) peaked back in 2020 with 1,070 yards and since then has gotten 849 (2021) and 801 last season. As Mike Greenberg pointed out this morning, contributing RBs have become "a dime a dozen" in today's market. Austin Ekeler (probably the lead FA RB right now) has improved over the last three years; 530 yards / 1 TD (2020), 911 yards / 12 TDS (2021) and 915 yards / 13 TDs (2022) and is still looking for a place to call his new home. Meanwhile Dallas just released Elliot with Pollard taking more of the carries and production. Chicago with Monty was in the similar position as Dallas was with Elliot. I'm never a fan of losing a player like Monty but in this case it makes sense when the bigger picture is considered.