I doubt London allowances will go. Its a recipe for staff agro and would reduce recruitment options in the long term. It's more likely employers will set new rules on home working and let people make their own choice about were to live and whether to apply for a job. The high cost of London office space will provide an incentive to maximise homeworking. Its absurd that many employers in Central London pay lower grade staff less than their desk costs!
It is already causing agro though. I have colleagues on the same band as me, who do very similar jobs, but are paid £4,000 per annum more than me because they are based in London. One of them lives a couple of miles from me and it is likely that they will actually work from my office when we return. So £4k extra because they notionally have a desk in London.
If, as looks likely, they will only go to the office once a week, why do they need an extra £4k per year?
I think it would cause agro to take the allowance away from those who already have it, but I can see it disappearing for new hires.
I don't think London allowances will go for jobs like retail where you have to physically be in London, but can't see office workers keeping them.
Maybe they will be replaced by a home working allowance to cover extra costs and compensate workers for loss of part of their home?