First off, let me rebuttal your question with another question: What do you lose if blizzard made r3 essences account-wide?
I never once said getting r3 essences were hard. They weren't hard with the original grind and they aren't hard now with the alternative grind. Lets just concur right there that there's nothing difficult about getting r3 essences. Now that we're on common ground, lets discuss the ethics of it. Yes I can do w/e content I want in order to obtain r3 essences, but why? Why have a grind in place for a system that'll ultimately get scrapped when the Shadowlands pre-patch releases? Hell, covenant abilities will be account-wide for alts to access without having to put in the grind.
I'll answer your question though. I'm a competitor. How I perform whether it's in m+, raiding or PvP, is of the utmost importance to me. I know atleast 2 specs for each of my alts and how to thoroughly play them. I pride myself on performing at a high level because I not only owe it to myself, but also to the others engaging in group content with me. Sure corruption effects play a bigger role in dps than essences do, but out of all the systems implemented in BFA, essences change your playstyle the most.
While you might see the r3 essences as an alternative grind, I see it as a missed opportunity. I don't enjoy assaults or much content outside of m+, PvP and raiding. Unfortunately for me, one of the easiest ways to obtain echoes is by doing assaults, which I don't enjoy. The worst part is that I can hop on an alt, get 460 ilvl in a week and barely have enough echoes for just 1 r3 essence.
So again, it's not about difficulty, it's about tediousness. Vacuuming your house isn't difficult, it's tedious. Doing the laundry isn't hard, it's tedious. I could rant all day about how stupid it is that r3 essences aren't account-wide for those that unlocked them already, but I don't think it would change Blizzards mind, especially here on mmo-champ. In the end, it wouldn't hurt you or any soul in the game if they were to make r3 essences account-wide with zero restrictions.