Ok Mr Sourpuss, tell me: How will the AR suddenly increase ?
As I see it, if you lose 20k Regulars, then the historic evidence is the reserves will go down as well.
The historic evidence is that the government of the day has chosen to cut both at the same time. Closing TACs/ARCs as part of the same drive for cost cutting by any means. But there's no hard-and-fast reason that a government couldn't choose to (or at least attempt to) grow the reserve whilst retaining the same size regular army, or while even shrinking the regular army. It just hasn't tried that yet, for whatever reason. As I mentioned before there are countries with bigger reserves than regulars, that proves that it is possible.
A government that wanted to do that has all kinds of levers it could pull to do so, if it had the will and the money. Free university for reservists for example. Or tax breaks for companies with so-many percentage of their staff in the reserves.