Again: Slot fees are paid once per flight, it won't harm you to pay it, once you fly your route you'll get that money within no-time. The increase of the slot fees is related to the amount of flights to and from the airport, more flights means higher slot fees and is irrelevant for the profits made by your Heron, it will take a bit more flight before you get your slot fee money back. Maintenance and fuel costs aren't that high, so that won't hurt your profit either.
I'm using the Heron now for a bit longer than 12 years and they are bringing in money on all flights I'm operating with them. I'm planning to keep a smaller number of Heron plane for a few more years at my HQ airport and give them their second D check because some domestic destinations still haven't enough demand for a larger jetliner (F28). The only two reasons why I will replace them is because of their age (oldest are now 17 y/o) and the aircraft limitation at my base airports (both base airports have already 100 aircraft operational).