Else why change the rules at all compared to SR5.
Perhaps they didn't change the rules at all compared to SR5, perhaps they just cut words from SR5 while hoping that that we should realize that it should still be resolved in the same manner.
For me it make most sense that the grip will remain during the attackers action.
That you can just spend an action to maintain the grip without taking any test at all.
That breaking free is something that typically only happen during the defenders action.
That you can deal damage while maintaining the grip.
That if you fail to deal damage you will still maintain your grip.
That you can attempt to knock the target down while maintaining the grip.
That if you fail to knock the target down you will still maintain your grip.
That you can spend an action to attempt to again restrain the subject you will in fact try to establish a better grip.
That if you fail the grip will start to slip and it will become easier instead of harder for the target to break away on their turn.
Since there isn't really anything directly contradicting this as written I will just continue ruling it like this.
Others will read it differently and think it make more sense to resolve it in other ways. They are free to do so

RAW
To be honest, RAW is actually pretty clear:
- Either keep restraining OR inflict damage. Not both the same time
- Either keep restraining OR knock down. Not both at the same time.
- To break away the defender only need 1 hit on an unresisted test.
Neither SSDR nor me think this is RAI.
RAI however is not very clear.
There are a lot of gaps and you can fill it with all kinds of interpretations.
You seen good arguments for different ways of resolving it. Feel free to pick one.
Or strictly follow RAW. That is also an option.
Anyway, I don't think I will debate this dead horse more for now :-)
Yeah, I’m dumping the test and going with major action to maintain restraint. Two chances per turn to break free is nuts.
Agreed.
And I am doing the same.
(but in addition to this I will also keep the option to take a test in order to improve on the grip).
Also IMO a big miss not to include Human Shield mechanics here.
In a sense I guess you have a human shield mechanic already build in.
If you shoot at the grappler then he take the test to avoid as normal. If he is successful then the victim take a test to avoid, but with a negative dice pool modifier of 4 dice.
Do you dare taking the shot? If you miss the guy holding the grip odds are high that you might instead hit the hostage.
If you also add on the Cover I, II, III, IV status on the grappler if he is aware and actively tries to hold the victim between himself and the shooter...... Bingo.
(but I agree that they should have added this as an advanced action to Grapple in Firing Squad - missed opportunity is missed opportunity)