There's nothing really theory about it KarinsDad, I'm playing high level, you're not.
The problem with YOUR theory is that first level spells are still very powerful at higher levels, and you have tons to burn. DC's still scale for those low level spells. Casting bless and bane every encounter is going to have a HUGE impact on the difficulty.
Tons? A 14th level Cleric gets 17 spells. That sounds like a lot, but not if there are 10 encounters in a given day. With only 4 first level spells, your Cleric will be dipping into 2nd level spells by the third encounter if he casts Bless and Bane every encounter. Is he really going to be putting Bless and Bane into higher level slots?
And, your Cleric is using up turns casting Bane and Bless instead of attacking (or healing after the fight). So yeah, he's giving +2 to hit (or -2 to hit) for three allies in order to not attack himself. Action economy-wise, not a huge gain (+30% more total chance for allies to hit -60% chance for cleric to hit). And of course, he has to be within 30 feet for this tactic to work.
I'm not seeing this as the mega-boost that you are. Sure, he could cast Bless ahead of time before combat starts, he could even cast it twice. But, that does not mean that every single day, those spells will not be wasted or quickly negated.
The problem with YOUR theory is the more casters you have in the party, the more quadratic the power level.
Not quite. You have yet to illustrate a double increase in power level at all with low level buff spells, let alone a quadratic increase. This is straight up hybebole.
The problem with YOUR theory is the monsters simply aren't designed like they were in 3rd edition, they're not designed to face quadratic casters, many of them are not spell casters themselves any more, and do not have all the special qualities (spell resistance, true sight, etc) that they used to. They certainly also don't have scaling saving throws like they used to.
No, but it's a piece of cake to double the number of monsters, or throw a ton of low level mooks into an encounter, or have a bunch of monsters show up from another room, or have more encounters per day, or have more traps, or a bunch of different ways to challenge the party, regardless of the rules.
The problem with YOUR theory is that players tend to hoard resources anyway then "Go Nova" for the boss fights (which also tend to last longer), so allowing them to quadractically buff stack means up to 4 times the amount of buffs/debuffs in those hard encounters.
I've seen a Level 14 Paladin Solo a CR18 Dragon with buff stacking via potions.
And who gave the party those potions? Did they make them themselves?
I do agree with you that a party that saves resources has a better nova option, but I don't care. PCs wipe out encounters all of the time. There are always more monsters per encounter / more encounters per day that I can throw at a party.
Your entire point is based on the predicate that the game is already balanced and that's your fallacy. The game is not balanced. Giving the PCs a single magic item can unbalance the game. I've played with players who are almost tactical geniuses who can shred an encounter and I've also played with people who could not think their way out of a wet paper bag. It doesn't matter. As DM, I can easily adapt the game to the players, and their PCs, and their tactics, and whatever rules we are playing. Can you?
You have zero experience at the higher levels, the potential for abuse is massive even allowing two combinations of concentration spells. I see huge potential for abuse with Greater Invisibility + other combinations.
That's what the ready action is for. Monsters can target ranged attacks at such a foe when he casts a spell. Greater Invisibility is powerful. It's just as powerful when the NPCs have it. If a player with a 14th level PC wants to use up a slot for Greater Invisibility to help win an encounter, great. HAVE FUN DOING SO.
Streamlined monster statblocks in 5e and the action economy simply isn't designed to handle buff/debuff stacking. Saving throws are not designed to handle buff/debuff stacking. Bounded accuracy is not designed to handle buff/debuff stacking. Breaking casters concentration when you're not a spell caster yourself simply isn't designed to handle buff/debuff stacking.
Nonsense. The game already has buff stacking because more than half of the sublclasses can cast spells.
But you know what, If you feel it's working? Good luck to you. For me, even as a player, I enjoy the challenge and tactical choice that the mechanic brings with it.
That's great for you.
I prefer that the players have fun over the classes have some type of artificial balance. The fighter at level 14 is still throwing out 3 attacks per round. PCs are already uber at that level.
I have an entire DMG and Monster Manual of stuff that I can do to challenge my players. I don't need the rules doing it for me. Maybe you do.