Technical limitations are one reason to cap, but more relevant to this discussion are the design limitations. Guilds provide buffs which make them both a social system and a gameplay advantage system. The social system seems silly to cap. Why not let everyone join one guild? But there are a lot of reasons to cap buffs.
Since Boundless has a player run economy the devs have to be very careful not to create chasms between the haves and the have-nots. Guild buffs are static cost which means the net benefit gets cheaper the more members you have. 100 players dropping 100k coins is like saying one week of a buff costs the individual 1k coins. Let that guild grow to 1000 players and now that same buff costs only 100c. A new player joining the game must either be absorbed into a mega guild or be at a huge economic disadvantage. Players effectively get to say who is allowed to play the game and who isn’t by using the guild system to enforce socio-economic classes. Eventually the game distills down to a handful of very strong guilds with a lot of power. Devs are smart to prevent this.
Some players recognize the economy of scale effect between guild size and static buff cost and ask for buff prices to scale with number of members. That removes the economy of scale and fixes the problem, right? Nope, this too will have adverse effects on the game. Realistically not everyone in the guild is paying for buffs. A few well-established guild members foot the bill and in return they get an army of guildmates to amplify their in-game effectiveness. The current system may be 1k coins per head but anyone can pay it. Under a scaling cost system new members must either be willing to pay their share or they effectively raise the cost for all other members. So this provides incentive for guilds to shrink and to consolidate around economic power. Again we end up with a class system.
I think the guild system is working fine as a social buff platform. I think the players who want the cap raised are jumping the gun and presenting a solution rather than a problem. It is much more beneficial to share what administrative problems they are facing so the devs can find a way to provide them which isn’t tied to guild buffs. The guild cap number should be seen as an invariant. I think the weird change last patch is something the devs will come to regret.
As usual I will place the blame for this situation squarely on the existence of alts. If one player meant one character this would all be going much more smoothly.