Agree with GOAT and PumaD on their points so I'm not gonna repeat them.
I just feel that last minute guilds are part of the GST experience they make it hectic, fun and dramatic. Having been in a last minute GST guild I found it to be one of my better GST experiences because no one expects you to get anywhere there's no arguing or bickering with whose a better guildsparrer. Also with there being so many last minute GST teams they seem to make up the majority of the teams in the GST with this new rule you'd be getting about 10 teams entering.
I just see that if you implement these plans it will reduce the level of competition drastically which would be a huge disappointment after seeing its level increase every GST.