You missed the Thordan Weapons!
Seriously though, the Sword of the Heavens also has a Crit/Det mix (albeit at a slightly lower value), so SE is already operating with duplicate stats across different end game weapons. There is a 5 item level difference, but it's not really a huge difference all things considered. Further, that Det/Crit mix does, in my opinion, put the ilvl 205 Sword of the Heavens above the ilvl 210 Relic. It's nice that the auto-attacks advantage Pld Dps when it comes to Crit. I don't really think it is the "best" dps option for Pld. Frequency in auto attacks is nice, but the drop in attack power nerfs the baseline. Also, I don't think putting faith in RNGsus for Dps (especially on the job that already lags behind it's peers) is not ideal if you compare it to a consistent increase in output. Plus, with the increase in SS, a Pld is likely to run dry faster, nerfing their Dps straight down to 0 when they have nothing left (which, admittedly, is only really a problem in fights like A4, where you have almost 100% uptime as MT) Given the alternative, it just seems like a strange choice to me.
As you said, they could have just as easily gone DET/SS (if they absolutely had to stick with SS) for a more consistent Dps output. The reason I find it an odd choice is because this seems to be what Pld's are designed for. They lack of any notable burst skills like Fell Cleave/IB or Carve and Spit/Reprisal... That is, unless they plan on buffing Spirits within sometime in the near future, but I hardly expect anything like this in the future considering they crippled Shield Swipe when they took it off the GCD (it would seem that SE does not want Pld Dps anywhere but down, which is fine... if they would just address that enmity issue...). That being said, I suppose it does vary depending on how the stats scale. If Crit has a higher scaling than Det, than it might make a notable difference. I'm willing to give it a test run or two, but given the overall dip in Pld stats when actually equipping the weapon I'm not expecting it to be all that different (if not outright worse) than the Antiquated weapon.