Much as I don't like how these lawyers are making money, they are entitled to be there and to do their job. Starmer has said so too, see the quote. What Sir Kier did before he was PM is relevant of course, but I don't think he's defending the law, rather defending the lawyers doing what they are paid to do.
I compare it to a defence lawyer defending a killer who is planning to plead guilty, but the lawyer has found that due process has not been followed so the charges get dropped.
As sammieuk1 said, the law could get changed to block this loophole. Or, the defence of a right to a familiy life could get accepted, and the whole family could get deported instead?