This. 10000 times this.
All characters, no matter the class, should have to spend time, money or other resources to gain the type of power generated by a 6th level spell. A spell slot (and maybe a component) is not, and never will be an high enough cost to justify that type of power.
Have wizards deal with weird and rare reagents, have warlock deal and do a favor for their patron, have the cleric sacrifice or quest for their god in order to gain access to the higher level of magic. If a fighter has to quest or spend a lot of money for access to better weapon/equipment, so should the casters.