It's not a matter of fairness. It's a matter that one of the mechanical methods that the designers added into the game to prevent abuse is, in the opinion of some players, too strict.
One cannot fly and go invisible. One cannot web and blur. Having such strict restrictions seems overkill. For players who enjoy playing wizards and have done so for many decades, not being able to cast fly and invisible (as one example) leaches a lot of the fun out of the class.
The fun of the class is in the ability to be versatile. Changing that to X or Y or Z, but not two at the same time, does not seem versatile.
But having the option of doing X or Y or Z is the very definition of versatility. Versatility is about the variety of potential options at any given time, not the momentary peak power those options give you. The ability to stack those options together is how the "quadratic wizard" thing comes about, as each additional option increases both the number of combinations and the maximum power in an exponential manner. For example, if you can do X or Y or Z, then you have 3 options; adding option W expands your choices to 4 options, but doesn't necessarily increase the power of the existing options X, Y, or Z. Change that to X and/or Y and/or Z and suddenly you have 7 options (X, Y, Z, XY, XZ, YZ, XYZ) with a maximum power level of all three combined. Add option W and you have 15 options (W, X, Y, Z, WX, WY, WZ, XY, XZ, YZ, WXY, WXZ, WYZ, XYZ, WXYZ) with a maximum power of all 4 combined. Add in option V and suddenly it is 5 options vs 31 with an even greater peak power difference.
It sounds like what you want is power
and versatility, but the two exist in a balance. It's like with tools, the more different things a tool can do, the less effective I expect it to be at each. My Swiss Army knife can be a can opener, a saw, a knife, a screwdriver, or a corkscrew, but it is not as good of a saw as a dedicated saw, or as good of a can opener as a dedicated can opener. It also can't function effectively as a saw, a can opener, and a corkscrew all at the same time. It's value is in the versatility in one package, I don't have to carry around a bunch of tools if I think I might stumble across a need for one of them, but if I know I'm going to have a need for a particular tool then I'm better off with the dedicated tool unless I'm constrained in how many tools I can bring. Wizards are the same way. They can be damage when you need damage. They can be buffs/debuffs when you need buffs/debuffs. They can be crowd control when you need crowd control. But they are not going to be as good of a damage dealer as a class dedicated to dealing damage, and they are not going to be as good of buffer as a class dedicated to buffing. If wizards could be the best at anything whenever they want, then the rest of the party might as well just stay home and let the wizard do everything on their own, and there would be no reason for anyone to play any class except wizard.
I remember in my 3.0 days, players complaining about spellcaster cheese I would throw at them and how unfair it was, when actually I was just using the party's spellcaster's tactics against them. If it is cheesy and overpowered for the enemy, it is cheesy and overpowered for the PCs too. Flying improved-invisibility hasted blinking casters with mage armor, shield, cat's grace, bear's endurance, and fox's cunning spamming summons, dispel magic, rays of enfeeblement, glitter dust, and fireballs. Dragons running mage armor, shield, blink, haste, and cat's grace to jack their AC up even higher. By the time the party reached the low teens, the enemy casters routinely had 10+ spell effects on them at the start of combat just like the PCs did, and it was a big part of the reason why we rarely went past level 15. Caster NPCs were far more powerful than non-caster NPCs of the same level. A L10 wizard should present approximately the same difficulty challenge to a party as a L10 fighter or a L10 rogue, not noticeably more or less.
