I think this is the biggest problem in this discussion.
People need to free their mind from the idea of guilds solely being a community of players that like to do all sorts of activities together. Guilds wont have the same “RPGish touch” as in other games (at least most of them), the ability to join several guilds has a purpose and is there (or proposed) for a reason.
This is probably how most of the guilds will be used (at least its how I´m gonna use them).
My gameplay in regards of guilds will probably look like this:
Join 2-3 guilds in the common sense of guilds (friends, people i like to frequently play with, skilled raider, etc)
And join/found 60 other guilds for every other minor build.
I don´t want to play with the members of those 60 guilds (besides of building ofc). I don´t want to grind dailies/experience with those people. I just want to collect some people for a build, build it and then leave it behind, ready to be explored by others.
And as such a player I’m absolutely fine with players taking their donated beacons with them when they leave (if the guild management is provided with a reasonable amount of time to react to the plot-fluctuation)
And I don´t say this as an assumption, this is exactly how I played in Minecraft (heavily modded to mimic what B< is going to be) and a LOT of people I played with played/felt in the same way.
I might be wrong, but I think this is the use of guilds @olliepurkiss intended with his system.
Honestly, this is the first dev-proposed system in a while that I can 100% support (besides of the part of not being able to leave a guild at any given time, which could (imo) easily be fixed by something like [this] (Beacons Survey Results - #37 by Vastar) )
But I also want to mention that I agree that guilds need to be restricted to not be able to instantly gain an unlimited amount of power solely because of the number of initially joining players. But there are other (better) solutions to this than restricting the creative space of people that just want to build pretty stuff together, it also doesn´t need overly complex “daily-quest like” systems.
With “Main-Guilds” hopefully being a thing a very simple solution might be possible: All your gained expereince also gets accounted for your main guild (with a possible bonus/multiplier if you play together with other players that have the same main guild), just like it´s handled with guild experience in most other games.
But the ‘guild-lvl’ rewards should definitely not be related to the (maximum) beacon count of the guild. Instead the guild-lvling could slow down the gain of power of a guild by tying the maximum amount of plinths allocated to guild beacon-space to the lvl of the guild. Or by giving central-guild tax advantages to high leveled guilds.