In the absence of rwy expansion, it is the only way that capacity can be marginally increased. However, the amount of congestion and delays will increase substantially, both on the ground and in the air.
You have missed out one assumption that's implicit in your scenario.
Increased
capacity can be applied in either (or both) of two different ways.
If
traffic is increased in line with capacity, in other words if LHR continues to operate at 99%, then delays and congestion would undoubtedly increase.
If, on the other hand, capacity is increased but traffic isn't (or at least not by the same amount) then the result is more resilience, because the airport is no longer operating at 99%, and so not only are delays and congestion reduced, but when they do occur the airport is able to recover more rapidly.
The latter, of course, is precisely what is being done with the current mixed mode trials. Having said that, the conclusions and consequences of the trial may well turn out to be as you forecast.