Yeah part of the problem here though is you already have at least one epic boon, and on a Rogue that is almost always the Boon of Combat Prowess, so it is almost impossible to miss.
I think you bring up good points and clearly you have concrete experience with epic play, so thanks for sharing that with us.
That being said, while I have no doubt that things happened exactly like you described in the epic campaigns you played, I think there could still be some alternative builds and tactics worth exploring. It’s entirely possible these are less optimal than the builds you witnessed, but I haven’t played them nor ran the math on them so the reality is I just don’t know. Still, I thought it could be an interesting thought experiment, so here we go.
Saves are a more common use, but a natural 20 does not always succeed on a save, and often won't succeed if we are talking about Constitution or Strength on a 20th level Rogue. A natural 20 will usually work on Dex, Int, Ch or Wis, but you also usually have Mage Slayer and these are covered by either Mage Slayer or Evasion, so how big a deal is a failure anyway?
If defenses are a common issue, perhaps the Boon of Fortune’s Favor, allowing to re-roll a save once per round, might be a decent alternative for a 1st Boon?
I think every Rogue I have seen played at 20th level, including multiclassed, have taken Combat Prowess as their first boon. You could game this and not use Combat Prowess to get the critical hit on a miss instead of a regular hit,
I don’t think that’s "gaming" the system. The Boon of Combat Prowess clearly says you "can" turn a miss into a hit once per round, so it’s an option, not an obligation. It doesn’t say that’s what happens on the first miss of the round or anything automatic like that. That being said, while I believe it’s a legitimate usage, your next point is still valid:
but then you are only really talking about making a normal hit into a critical once a short rest because your miss would have hit anyway.
Moving on…
You could also take a different Boon, but on a character who relies on attacking and making one big massive attack, that is a huge downgrade at level 19.
Is it though? I guess it depends how hard to hit the enemies are, and thus how often the Rogue misses… but assuming a TWF setup yielding 3 attacks per round, yeah, there is certainly a good chance you miss at least 1 of the 3, and so Combat Prowess has a decent chance of kicking in regularly. But how likely is it that you miss all 3, and that Combat Prowess is the determining factor for landing the SA damage? Ultimately, that seems like what would matter most, right? That the SA damage lands and thus at least one attack lands. Going from 2/3 to 3/3 attacks that land is nice, but either way the SA is locked in already, so the incremental damage from the 3rd attack is not stellar (assuming no special stuff like an enspelled item of CME and a maxed out Moonblade dealing +3d6 Force damage).
What if we leaned into the 20s and went for the Boon of Irresistible Offense instead? Assuming 21 Dex, that’s 21 extra damage on a nat 20, which is not that far off from the 35 extra average damage of a 10d6 SA. So this Boon would mean that a nat 20 translates into an extra ~2/3rd of a SA worth of damage for the round. If you go crit-fishing with advantage on 3 attacks (and my math is not failing me) there’s a ~26% chance to get at least one 20 out of those 6 rolls. This is in the same ballpark (maybe slightly less) than the 1d6+6+magic weapon bonus extra damage you would get from a 3rd (non-SA) attack that hits instead of missing…
Even at level 20, turning a miss into a crit once a short rest will not make up for just plain never missing.
Ultimately, I agree that it’s not the most impressive capstone. I would argue it’s better than some (e.g., the Wizard’s) but it’s not stellar.
I was not trying to deny that, but rather try to explore whether Combat Prowess was really a must have versus some other options. Not for any character but, rather, specifically for the Rogue, which has this dynamic where it is very important to land one attack for SA damage, but landing one more non-SA attack is not necessarily as significant.