TL;DR everything is going to continue as normal.
So what's next?
Is the Beta going to be extended (how is that decided and by whom?)
Yes. Sites may stay in beta for 2-3 years before any decisions may be taken. The Community Team reviews every beta site every 2-3 months, and looks for any problems or issues. If there aren't any ominous posts from the CM team, that usually means the site is doing fine, and is progressing towards a graduation.
What will happen if the Beta is not extended?
This will be a long time away, if at all it gets to that.
If the Beta is extended, how can we improve Arduino.SE achievements to meet SE expectations?
The statistics are not absolute criteria. A site may be kept in beta, even if it meets all the criteria; a site may be launched even if it meets 3/5 of the criteria. The numbers are there just to guide the community on some generic goals. The CM team knows and understands that each site is different, and they look for continuous stable growth, rather a set 3-4 parameters. The graduation process is very fuzzy.
However, sites should try to stay in the green on atleast 3 to 4 of the 5 parameters shown on A51.