EzekielRaiden
Follower of the Way
Well, let's look through the list of 3rd level Wizard spells.Fair enough. I am just having trouble imagining a concrete example of something that might occur in play. But, again, the overall point is good.
clairvoyance -- who needs Perception or various means of trickery to listen to conversations when you can just make an invisible sensor that can hear (or see, but not both) anything near a chosen spot?
fear (or possibly enemies abound) -- functionally an Intimidate check with few to none of the downsides
feign death -- literally undetectable pretend-dead status ("the target appears dead to all outward inspection and to spells used to determine the target's status")
fly (or perhaps gaseous form) -- I don't think I need to go over the massive ways that this completely outshines any Athletics/Acrobatics stuff a Fighter could do
tiny hut -- Literally has a thread on here right now asking about folks' exposure to it and whether it's a problem for them (the author of that thread was skeptical, but still--that the question even needs to be asked indicates the potential is there)
phantom steed -- who needs Animal Handling? Literally just summon a supernaturally awesome horse-illusion, no concentration
sending -- perfect (for on-same-plane), instant, unblockable communication with return message, extremely useful tool that obviates issues like "how do we let our captive ally know what our plan is?"
speak with dead -- not as reliable as it could be, but still: why bother mucking about with holding back, when you can just kill your opponents and squeeze info from their corpse instead?
tiny servant -- Animated objects are amazing infiltrators, distractions, or helpers in many ways, obviating the need for stealth or sleight of hand in many situations
tongues -- eliminating any benefit from characters studying languages since 1st edition D&D
water breathing -- no need to bother with swimming if you can just trudge through the water normally, obviating various benefits of Athletics
That's just one spell level for one class. And 3rd level spell slots are cheap at high level. You're not using those for damage anymore, unless you're really strapped for stuff to do. Sure, no single Wizard can cast all of these spells unless they've spent a ton of time buying and scribing scrolls. But the point stands. There are plenty of ways for Wizard spells, even low-level ones, to turn a complex and effortful non-caster's plan into "oh, well let me just solve that for you right away", or to change a built-up effort by the player (such as intentionally acquiring lots of languages) into "well I mean I could've just cast a spell..." or the like.
For anyone who wants high-level 5.0 to work well, you need to keep extremely careful attention on both how many spells your spellcasters can deploy in a day, and what specific spells they have access to. Tailoring challenges so that your martial players feel like their choices have been smart and rewarding, without having "oh, I have a spell for that" happen half the time or more, is a serious concern. I don't have a good answer other than shouldering that burden yourself.