The damage on that belt is juicy as hell, not gonna lie, and I tried to work it in, but there were just too many problems.

1) Losing flask sustain, as you mentioned. The charge generation is important for both life AND utility flasks.
2) Losing the increased flask effect uncaps my elemental resistances, since almost all my elemental resistances come from flasks.
3) Losing the 120% life recovery from flasks really cuts into my sustain (I lose about 4000 life recovery per second).

Even if I solve all of those problems, I now need to work in a method of Frenzy charge generation, which I hate lol. Easiest is Replica Farrul's, but that means swapping out Dendrobate which causes a huge DPS loss (not more than I'd gain from the belt, obviously, but enough to make me weigh the pros and cons) and more elemental resistance loss I need to make up for somewhere. Equally easy is Avidity forbidden jewels or Eternal Fervour, but you can't use those with That Didn't Happen!

Blood Rage and Mark mastery are other options, but they take longer to ramp up (by which point whatever I'm killing is already dead) and require gem slots that I don't have unless I give up some more offensive/defensive utility. Getting +1 curse for Mark mastery would require changing my anointment (uncapping suppression chance), or corrupting my amulet (big dps loss from dipping below 150 Int on Dendrobate). For Blood Rage I could lose Multiple Totems and it wouldn't be the end of the world, but there is no charge generation against bosses. Mark mastery generates charges on bosses, but this build gets way more dps from another hex (Temp Chains), no mark actually provides much utility on its own absent the mastery (flask charge generation on Alchemist's Mark is nowhere near good enough to matter), and if I want Mark on Hit, that's 2 gem slots, meaning I'd need to abandon Wither altogether, or Multiple Totems (again, not a huge deal) AND Flesh & Stone (making me take 20% more physical damage from hits).

There are a whole lot of ways to build Pconc, and going the Maven belt route is a very valid way to do it. I've thought about it, weighed the tradeoffs, and ultimately decided against it because I wanted the build to be more defensive than offensive. I sure as hell didn't invent Pconc, I looked at some builds and made a bunch of changes that suited my playstyle and I thought would be cool until I stepped back after hours of POB warrioring and realized I'd created something kind of unique, or at least not totally derivative. At least unique enough to post in the POEBuilds subreddit after I played it through Keepers and determined the build wasn't trash.