The problem with a sliding scale by month is how it affects small businesses differently to large businesses and is dependent on what sector you are in.
For example you are a very small restaurant or pub with no outdoor space and no real way of maintaining social distancing and maintaining enough business to be open. The opener has little capital to fall back on and there are still outgoings eating up what reserves of cash the business has. It has no real way of generating an income until all restrictions are lifted.
Contrast with the likes of Virgin Atlantic whose owner most definitely has the money to pay to furlough staff but doesn't wish to.
And a well managed business that values its staff and can partly open in July and therefore can start generating income so may be able to support the staff that can't currently return to work.
The first may have no choice but lay off staff even though they don't want to. The second example might just be tossers to whom money is more important than their moral responsibilities to their staff.
Therefore a tapering of furlough in sectors which are hamstrung for the longest is particularly difficult and may not be enough to mitigate redundancies at all for very different reasons.
I hope that smaller businesses are generally supported better than larger multi nationals that are registered in tax havens. So the government will provide different levels of support based on a) your size b) your sector and c) you domestic tax status to encourage businesses to take their responsibility to staff better in the long term and to help those firms most at risk of not being able to survive otherwise.