So, lets make some projections about just collecting mythics. Legendaries and Runics and Arcanes are goals that are likely to be completed before collecting all mythics if you started post 2.1 (or now post 3.1) because you get them incidentally due to the gigantic amount of keys you have openend and time you have played to achieve this goal if you need to collect retroactively. This, arcanes and runics are effectively zeros toward this goal, as are legendary troops.
To do this, I need to define Mythic Purchasing Power (MPP). I define this as the overall contribution or chance toward getting a new mythic.
From the table, for relevant mythic hunting items per LT, we have about:
7.5 glory keys - 0.00075 MPP
2.94 gem keys - 0.00294
1.06 event keys - 0.00106 MPP
103.14 glory - 0.000573 MPP (converted to event keys)
0.01 mythic troops - ~0-0.005 MPP (varies depending on how many you need to collect and how many are in the game)
For a total of 0.005323 - 0.010323 MPP per LT. As you can see, the relative value of a legendary task is going to vary a lot dependent on how many Mythics you currently have, since the ability to get a duplicate from the legendary task will severely diminish its overall value. In a perfectly balanced guild, 1 mil gold put toward legendary tasks while everyone else also puts in 1 mil gold nets ~0.15-0.30 MPP depending on your VIP level and level of progression, but those aren’t realworld conditions. However, at minimum, you need to be shouldering at least 250k of the cost of every single legendary task in your guild before you consider pulling further donations and them toward guild keys. 250k of each task, total, not just the ones you personally donated to, but an average of all of them over an extended period of time. And you need to be donating the full 1 mil, by yourself, for gold keys to be better by a very thin margin if you are non VIP and don’t have any mythics.
I feel I can’t stress these last point enough. I can’t think of any real-world scenario where gold keys would be “worth it” above legendary tasks, even post nerf.
Edit: revised for half a percent Mythic drop chance rather than 1%. This does not affect my lower bounds worst case scenario calculations, which assumed all current mythics collected and therefore ignored this drop entirely.