In 10 years time, demand for LHR slots will likely be considerably greater now, balanced of course by a likely R3. A normal airport should have peak and offpeak periods - LHR right now has only peak (except for 10 pm on Saturday night)
By thd time R3 opens, peak time slots should still trade at a high price - the only difference is we might see space at off peak times for a few extra not-so-wealthy destinations. If Easyjet want into LHR and do not get slots from an open application to ACL in 2025, they will have to pay heavily for commercially useful slots