I agree that Haste Samba merits are a lot more niche than they used to be... but I don't really think the alternatives are compelling enough to warrant changing merits to Building Flourish. There are still enough practical current situations where 5/5 Haste Samba IS useful that you probably want it in your toolkit.
- Sheol Gaol without subjobs (no DW subjobs for jobs like COR BRD BST, no /SAM)
- Allows DD/SAM or /NIN (for those who need to DW) to change to DD/DRG for stronger WS and better enmity control while still capping delay reduction
- WAR fencer builds (pretty popular these days)
- SAM or /SAM who pops Seigan at some point
- DRG in an oshit situation where the wyvern died and timer is down (not unreasonable to think that might happen in major AoE fights, and it's nice for you to be able to cover for that as the DNC)
- Niche stuff like dealing with a slow aura (e.g., Dyna Bastok wave 2 boss)
If you're using DNC more for solo, you never play with any Naegling WARs, and you don't bring DNC into Gaol with any jobs that normally need subjob to hit cap, well... OK, maybe for you it makes some sense to go for Reverse + Building Flourish merits in Group 1. For most people, I'd think the long standing advice of 5/5 Haste Samba (and 5/5 Reverse, the truly non-negotiable one) is still the better call.
Or if you know exactly why you want it, and don't mind reconfiguring merits, go for it. I'm just hesitant to say that should be a general recommendation (especially for DNC who don't know precisely why they're changing for a specific situation).
Building flourish merits are almost useless too. You will use BF mostly for Rudra, which means crit rate and acc is pretty much useless. So you only get 5% attack.
Not seeing how crit rate +5% is useless on Rudra's, though I do think it isn't enough to outweigh the benefits of Haste Samba merits for most DNC.