Monsters are too easy
In Master of Magic, fighting monster lairs was very challenging and dangerous. There were flying units with ranged attacks (sprites) that could destroy a weak army very easily. Zombies and Ghouls, the basic death units could also punch a whole in your forces in the early game. If you were lucky you could fight a single guardian spirit or war bears.
In that game, you get 1 EXP / turn, and 2 EXP/battle, with a limit of 1 battle per turn for each army. This meant that it takes many turns for your heroes to level up, and they start very weak. So this forces you to build up an army of trained units to escort your heroes to kill the difficult monster lairs. Sometimes you would have to wait 50 or 100 turns to kill some monster lairs because you needed strong ranged attacks or powerful melee tanks. Some monster lairs are almost impossible to defeat even in the end game with the best possible army.
Right now, my starting sovereign partnered with a champion can kill about half the monsters on the map even at level 1. Once you get some armor, a shield, a good weapon, you can take out about 75% of the enemies. Then you just get a few tough armies that require better gear and higher level champions, but eventually you can kill those too without any trained units. The problem is that you don't need trained units to kill monsters, and that killing monsters gives you items for your heroes that makes them stronger, creating a feedback loop.
Fallen Enchantress is a 4x game primarily, not an RPG with 4x game elements. The RPG elements are interfering with the strategy aspect of the game and turning it into a grind. You can ignore the RPG elements and play a pure strategy game, but you are being sub-optimal when doing so. The only way to really fix this is to make champions less important. You could increase their wages to match their level which would make them less cost-effective compared to trained units, but that is a mediocre solution. I really think that there should be monster hubs which are very dangerous and spawn monsters that will directly target your cities and outposts.
This would change monsters from being a fly swatting type of activity to a real strategic part of the game. You have to avoid monster hubs until you are ready to face them. You can't build cities near them so there is real pressure to clear them out. Clearing them out requires you to acquire military technology and produce lots of units to aid your champions. So this would create a whole new interaction between trained units, cities, and champions that feels much more fun to me.
I still play Master of Magic and have fun doing so. The main reason is that it is fun to build a stack of units to support your hero and go and clear monster lairs. Sure capturing cities, building my own cities in nice locations is also fun, but not enough to keep me around. If you want people to play Fallen Enchantress 10 years from now, I really don't think you have addicting gameplay to warrant that. The problem is squarely with the conflict of RPG and strategy mechanics and you need to find a way to make them synergize. There are other issues that make the game less fun for me, but I think this is one thing thats easy to explain and fix.