The mistake is yours because the existence of such monsters does not fix the problem itself. The fact that there are monsters that go higher does not change the fact that thanks to bounded accuracy the underlying math for all of it is hot garbage if the GM has a larger or higher leveled group than bounded accuracy assumes & the tools to provide that GM the ability to make finessed nuanced changes targeting the needs of their table have been removed. It does not help if monsters like tiamat exist that need a twelve or fourteen rather than a six or eight...
tiamat has a +19 to hit and a mere 615hp. A five player group will pulp her in a couple rounds. A high level group is going to have at least +1 weapons if not better. If they all 5 players hit it works out to just shy of 150-200damage per round with regular attacks & cantrips giving her a lifespan of three rounds
if the party doesn't bother to nova. If the 5 players take the effort to nova she will be lucky to make it through the second round. Meanwhile her +19 tohit makes her likely to hit every round for a grand total of 76 average damage per round spread across 3 attacks(24 24 &28). A 12 con wizard will have 102hp leaving them with room that is almost certain to be able to take one or two more attacks from her before the third drops them to needing to make a death save on round 3. A warlock(d8)/5 paladin(d10)/6 fighter(d10) or barbarian(d12) will have 123 144 & 165 respectively at the same level 20. Level 20 is indeed higher than nearly anything but one shots will go but those numbers do not improve much as levels go down because she's probably only going to last one or two rounds at best.
- d6 at L10 & 12 con: 52 hp
d8 at L10 & 12 con: 63hp
d10 at L10 & 12 con: 74hp
d12 at L10 & 12 con: 85hp
- Tiamat is almost guaranteed to drop one PC every round & can choose to execute tha PC or drop a different PC. next round. With a +1 weapon a PC is going to need a 15 to hit on any attack & she is guaranteed to TPK the party if she does not allow them to flee or find herself on the receiving rend of shenanigans.
- Even giving players +3 armor & +3 weapons is unlikely to change those results because her AC is pegged against all attacks rather than just the second or third onewhere being able to hit with the first or second with a much more reliable 10 or less depending on which attack is pegged to need the 15.
- d6 at L15 & 12 con: 77hp
d8 at L15 & 12 con: 93hp
d10 at L15 & 12 con: 109hp
d12 at L15 & 12 con: 125 hp
- These PCs would need a 14 to hit with a +1 weapon but have the HP to almost certainly soak one or two full rounds & can get advantage from flanking, bless, a helpful familiar, or whatever & things go back to one towards her lasting one to three rounds
- d6 at L20 & 12 con: 102hp
d8 at L20 & 12 con: 123hp
d10 at L20 & 12 con: 144hp
d12 at L20 & 12 con: 165hp
Attack chains are all or nothing making a monster completely helpless against PCs or they are able to utterly dominate with only a razor's edge of probability spread across too few rounds to average out the highs & lows. Without no longer present subsystems like SR to encourage casters to engage in force multiplier abilities like buffs & debuffs that might turn the tides those spells are subject to the same +9 to +19 save mods as nukes leaving casters with little incentive to not go for max DPR too.
Tiamat might look like an extreme example & I'm sure that we could easily find monsters with each step along the 20-24ac spread but every single one of them will ride on that same unfun razor's edge where the GM is walking the line between a snooze of a combat with a giant bag of HP or a straight up slaughter the PCs never stood a chance on. This is entirely because the GM was robbed of system mechanics that expanded the wiggle room by providing a safety margin for good/bad luck & tactics to matter within the number of rolls likely to exist in a single combat. The razor's edge of bounded accuracy does not provide room for that margin once the group exceeds bounded accuracy.
The mere existence of monsters not bound to the table in question & other dials does not make them a solution for a problem they do not address. Bounded accuracy is too tightly bound for a specific groupsize of a specific level & it breaks down into major league champions vrs middle school playground pickup team or rodket tag with unlimited ammo