This is speculation, but Yoshi's comments regarding the strength of invulnerabilities and the introduction of superbolide (with it's cooldown timer matching that of HG) gives us a bit of perspective. In their view, Holmgang and Hallowed are overpowered, and they would nerf them were it not for fear of backlash for keeping them unchanged for so long.

Superbolide then sets a new precedent. This is what the devs now consider in-line with balance. Something that's stronger than Holmgang but weaker than Hallowed Ground (as far as effect goes) is deserving a cooldown of 7 minutes. I can only imagine if devs had their way, they would increase holmgang and hallowed ground's cooldown.

However, this new standard also increases the value of the shorter invulerabilites. LD's 5 minute cooldown is now given more value because we now understand what value is given to cooldowns. In exchange for an invulnerability that requires more team interaction, we are given 4 more seconds of invincibility than holmgang, along with a 10 second prior buffer.

In short, LD may come with its issues, but it comes at a fair price as far as cooldown length and power is concerned.