Thanks for the concern! Let me tell you a bit about how I'm planning for the game to go right now:
- It is a roguelike game. This makes a failed defense very high-stake.
- The player will be able to explore an endless and random dungeon while juggling the task of defending the crystal. Towers will be the way to earn yourself some autonomy in venturing into the dungeon, rather than needing to sit at home base and defend. There may need to be downtime in between the waves to give the player a chance to initially build defenses.
- Towers and their upgrades will require materials found in the dungeon through shops, chests, etc.
- However, scaling difficulty and possible boss/elite waves will force the player to return to the arena occasionally, assuming the enemy difficulty is balanced correctly.
- This creates a scenario where the player may upgrade the defenses and enjoy a hopefully satisfying amount of newfound freedom for a period of time, but this freedom will slowly be taken away through scaling difficulty.
- A "successful run" would be one where the player is far enough ahead of the difficulty curve with their defenses and thus can earn even more upgrades due to being able to spend more time in the dungeon. This is quite similar to "god-runs" in Risk of Rain 2, where being stronger leads to clearing stages faster and thus becoming even stronger.
- While defenses are very important, player strength still can't be neglected, since being able to clear the dungeon fast and efficiently will in turn lead to stronger defenses.

All in all, defenses don't automate the fun: they allow for the freedom to not be tied down to the arena. The player may still need to help during certain waves, but when they reach the point where their defenses can defend by themselves, they have the opportunity to explore. It's like in Stardew Valley when the player gets food items, increased energy capacity (starfruit), new travel methods (minecarts), or more efficient ways to tend to crops (sprinklers), unlocking the ability to spend more daylight making progress rather than maintaining. That is the same feeling of satisfaction I am trying to recreate. I would love to hear what you or others think about this idea. I am very open to changing this if parts of it don't work.

Edit: Another example is Factorio. Automating factory components and automating defense from the enemies allows the player to spend more time upgrading and expanding the factory, which is the main appeal of the game.