it's reasonable because

you must be able to demonstrate that your intention at the time of gc is to remain with the sponsoring employer indefinitely.
If you have used ac21, you have demonstrated this so you don't have to prove your intention again.

There is no required amount of time that gc holder must remain with an employer, but 6 months is a good time for "indefinitely". However, I believe eventually all gc holders got their citizenship even they left company within 6 months