....This is why I got away from the homebrew spell points system we'd been using for decades and went back to limited slots - the s.p. idea works great at low level but just gets too powerful at higher levels when a caster can put all their spell points into casting the same low-level spell over and over.
Your experience with this is 100% different than mine. It is exceedingly strange that we'd have such fundamentally opposite experiences.
Within combat at high levels - even in 5E (as opposed to 3E or earlier) - casters do not run out of spell slots. Warlocks can, and there are occasional builds designed to use multiple spell slots per turn (soradin that uses a slot on a quickened spell, 2 more slots to smite on a multiattack, and then one more to smite on a reaction attack can get through 4 slots per turn - they can run out over a few encounters). It is this opportunity cost dilemna that keeps it from being overpowered in combat.
The extra slots tend to be used outside of combat - and that is fine. It gives the PCs more of a feeling of being a powerful spellcaster for out of combat endeavors. You end up with less of the, "I need to save spell slots for combat just in case, so I won't use that utility 1st level spell, even though I prepared it. You never know when you'll need to cast shield, absorb elements, silvery barbs,...." This keeps most low level utility spells from being used, even when prepared.
More reasons to know this is all fine:
Wand of Fireball / Wand of Lightning Bolts give nearly limitless evocations. This is an item I see PCs drool over when I give it to them ... but unless they get it at below ~8th level, it tends to not get used that much. It does in psuedo combat situations to wipe out non-threats that could be eliminated other ways (or to destroy barriers, etc...)
Warlocks already get unlimited use of low level spells - and at much lower level than I suggest giving them. Unlimited Silent Image, Disguise Self, Mage Armor, False Life, Speak with Animals, or Detect Magic an infinite number of times at 2nd level. You even have 5th level warlocks that can cast sending an infinite number of times - and that was added down the road indicating they thought that expanding the options for warlocks was appropriate. Other spells become available at higher level. If you look through how often these spells are taken - it is not a lot. Despite the limited spell slots of the warlock, these invocations are often ignored.
