How could you do this to AST?

[Rilifane]

Ok and the 700 regen will never stop you from 1 death mechs like a shield has the ability to.

Let me highlight for you what you so skillfully ignored.

If the shield doesn't prevent death, then shielding for 300 is less efficient than regening for 700.

If shielding isn't absolutely neccessary even after exhausting all other options to survive a hit, it's a waste. Period. You can still prefer it but that doesn't make it objectively better.
And there isn't a single mechanic that will be a guaranteed one shot regardless of gear level and with mitigation. I'm pretty sure you as an experienced savage raider have noticed that people don't just drop dead from ful HP because of one raidwide.