I find that a lot of players are carrying around a lot of assumptions about what Illusions do based upon what happened with them in prior additions. I suggest that you take a look at the actual language in the player's handbook relating to Illusions and the actual language of the illusion spells and reconsider what it takes to realize that there is an illusion there. For the most part, it is just pure role-playing. You don't get to roll a save to try and determine that it's an illusion. You just have to role play out what happens when a wall suddenly appears in front of the monster. Does the monster think it's a wall, do they know about Illusions and assume it is fake, or do they guess something else entirely is going on? What they believe, and what happens when they ty to interact with it, is different for the different types of illusion. And once you have a strong suspicion that it might be an illusion, you still can't see through most illusions. You have to investigate it in order to be able to see through the illusion. That allows the allusion to still provide concealment even though someone knows what they're dealing with this an illusion.
Some illusions follow a script you specify when you cast the spell. Phantasms tend to take place within the mind of a foe and adapts as the foe expects them to adapt. Other illusions can be manipulated by the wizard as they unfold so that the wizard can decide how the illusion evolves. A creative player can look at these rules for illusions, especially when enhanced by the illusion School benefits, and can be insanely effective if the DM is accommodating.
Finally, you have to consider what you can use allusions to do in D&D. Well they can be used in combat, they can also be used out of combat for a wide variety of things that might give you an advantage and a subsequent combat, or my help you avoid combat, or might just get you a reward of some type without ever even needing to worry about a combat. Illusion is just fine as it is.