I wouldn't expect STN to gain a significant growth of pure-cargo traffic well into the future. There's a bit of an uplift now while passenger growth is constrained due to Terminal capacity.
The airport has planning approval to increase to 43mppa within a 270k aircraft movement limit. They're currently doing about 29mppa with approx. 200k movements. On basic workings, that means 145 pax per movement (I know that's not entirely accurate because cargo and GA are included in that 200k total). The future 43mppa though on the same logic works out at 160 pax per movement. Yes some of that will be achieved by aircraft upguages to 321neos and MAX10's, but it won't allow any significant increase in cargo or GA ops, rather the contrary.
Longer term the pure cargo traffic will be priced out and likely go to locations such as EMA. Of course STN may also be using it as a tool now to incentive long-haul passenger traffic in the future, demonstrating there's also a significant belly-cargo potential should they do that.
Emirates for instance when they started passenger ops in 2018 I believe we're sold very heavily on not only the passenger demand from NE London and the 'London-Stansted-Cambridge' corridor, but also the cargo demand, which their 777's also are better suited for than the A380.