Kicking one person isnt the issue here. Most guilds can kick one person right now and barely notice the difference in decay, unless they were literally right on the cusp between size levels.
What the system attempts to curtail is inviting a ton of noobs from the harbor, leveling the guild quickly through lots of repetition of harbor quests, then booting 95% of those members and keeping the guild high level.
With my suggestion theres no reason for regular decay. Kick one or two people and youre fine. Yeah you deal with a penalty but with meaningful contribution it doesnt really hit the guild that hard. Kick like 25+ people in the same week, and watch the level of the guild drop.